Andrina

I've never labeled anyone my best friend, as I know it's normal for friendships to evolve throughout the years, and especially a lifetime. A great metaphor for that is "Never keep all your eggs in one basket."
It's okay to feel disappointed about a friend not putting in as much effort as before. How I operate is that I let actions, or lack of them, speak for themselves and I don't go into discussions, calling out a person. I would only have a discussion if said friend asked about a change in my behavior or if they've upset me in some way. And then I'd be honest.
If I don't feel like I'm a priority, then I'm not. If a person wants to be in my life, they will make an effort. If they don't, I let them fade away, and I have done that when friends have distanced themselves. I always leave the door open and don't hold grudges.
It's best to have the mindset that there is no rule saying a person is bound to you for life because they are presently your friend. Sometimes friendships work for a time, and then they no longer serve the purpose they once served, and that's okay.
I don't know if you're male or female. If female, it might be that since he now has a gf, even if you set them up and your friendship is platonic, that he feels it's no longer in the best interest of his primary relationship to have a female as a best friend. It could be that he only finds local friendships valuable, and doesn't want further invest in someone who is moving away. Who knows? Maybe he prefers to spend leisure time in new ways--new gf, new life, new activities, new friendship groups. It doesn't matter in the end.
Start being excited about your move, and looking into social groups you can join once there. 
My advice about him is to keep the door open, and just give equal effort to his, whether it be none, minimal, or builds to a satisfying level once more.

Sure, well, if you'd be happy building a life in Colorado regardless of whether or not a relationship happens with Kay, and you're not leaving some high-powered career not available there, why not move? 
Even if you dated her and it didn't work out, or if you read her wrongly and she's not willing to date you, you could still build a nice life close by to your sibling, who's your buddy. Siblings are who we know the longest while on earth, and it's great you have one you're close to.
It's probably actually good that you haven't begun something with this woman, and then months later say you're up and moving to be close to her. Because even as that would be the goal for new partners, it adds pressure to the person in that situation whereas they might think: OMG, this person's moving all this way for me. What happens if after two more dates, things fall apart?
Just keep with realistic expectations, that your being happy there isn't contingent upon Kay. If it happens that you two date and it works out, it'll be a bonus. And if down the road you're dating and she asks if you moved there for her, you can word it just like I've said here, such as, "Yes, I had hopes, so it was a bonus to me moving to a place I enjoy, and to be closer to my sister."
Things look promising with her, but if she's not your fate, another lovely lady will love to be your number one as you sound like a real catch. Good luck and let us know what happens!

I'd sit down with him and explain things, speaking of things you want and how you feel, using "I" sentences instead of "you never."
Nobody can argue with you about how you feel, and avoiding "you never" statements will be wise to avoid your partner feeling attacked. 
Such as, "I want to feel more connected to you, and want us to hold hands while we watch t.v."
"I feel most loved by you when you caress my back when we hug. I could use a hug now."
"I've initiated sex the last two times. I'd like it if you also regularly initiate."
When a person cares, they will want to please you. You have to be concrete in your request. I know it's not romantic when these changes first happen, but if he puts in effort, try to feel warm and fuzzy that he cares enough to improve. And then lay on the appreciative words for positive reinforcement. Hopefully, a new pattern will emerge.
If he doesn't care, nothing will change, and that's when I would say, "I'm done."
I'm not one for playing games, but sometimes people need a shake up and a wake-up call. Why not let him see what life will be like without you? Take up a new hobby. Go out with girlfriends more often. Let him miss you. Show him you'll be able to move on with a fulfilling life without him if need be.
Good luck and let us know how it goes.

To me, that sounds like sabotaging the relationship, but in a different way other than infidelity. Make sure you really are serious about continuing this relationship and not just staying because she's been a major part of your life from youthful days and you're merely used to her being around.
Questions to ask yourself to know if you're really serious about becoming the mature adult it takes to build a beautiful life with someone. 
Am I willing to never drink again, knowing I lack good sense when I do so?
Am I willing to stop building friendships with women who flirt with me and who, if I was single, would want to date? This includes not communicating with these women by text nor any social media.
Am I willing to delete and block anyone who crosses boundaries and is bad for my relationship, including the woman who took advantage of me?
Am I willing to solely behave in ways whereas if my gf was a fly on the wall, she'd be happy about what she observed?
If you're unwilling to abide by any of those things, do your gf a favor and let her go to find someone who is worthy of her.
If you're serious about stepping up, good for you and good luck.

I'm of the sort that whether it be for romance when I used to be single, or with friendship, you don't do a lot, if any, double-asking. You ask them to do something and if they can't, the ball is in their court to ask you to do something. If they don't, there's your answer and you haven't wasted any excess energy on someone who doesn't make you a priority.
I know I had a 10 year friendship end after the only time she reached out, 3 times in a year, was to ask a favor. I saw that with others over that period, she did manipulate people and use them, though had a lot of good qualities as well. Our friendship worked while it did, and then it didn't.
I didn't have any discussion with her about what I felt as to me, her actions spoke for itself. We let each other fade away. If she did ask why I stopped reaching out, I would have told her the truth. But since she didn't, to me she doesn't care and I don't beg people for attention or try to coerce them to change.
In your shoes, I'd do just what I mentioned above and let the cards fall where they may. If you feel like he's inviting himself over for free tea and alcohol and you'll be bitter if that happens, just say you're busy. If he values your friendship, he'll figure out that a real invite to a fun event for quality time is something you're more likely to say yes to. If he doesn't ask you to do anything, don't make any effort yourself. 
It's a shame if the friendship fades to nothing after so many years, but you shouldn't be the only person driving the train. Good luck.

Mature, decent people with good ethics end one relationship before beginning another. She's crossing relationship boundaries. That's how she operates. You're attracted to her and so you're overlooking that huge red flag waving in your face. What she's doing to her man is what she would eventually do to you if things even got that far.
And she's been with the guy a whole six years. Even if she broke up with him today, do you really think she'll be fully ready for a new relationship tomorrow? You don't know much about relationships if you don't know it'd likely take her a full year to be good and ready--a person who has gone through all the stages of healing after a longterm breakup definitely need that solo time.
You're a fool if you don't tell her you've had second thoughts about staying in touch since she's taken. And then delete and block. I don't even think she's worth pursuing if she makes herself single. 

When you're older, you'll see that she's done you a favor by easing her way out of your life. A person who drops out of college and has to be cajoled and motivated to seek out a career won't be pulling their weight financially in a relationship. From what I've witnessed throughout my life and the people I know, a person's work ethics and whether or not they are go-getters usually don't change just because they get older. At her age, I paid for my own community college where I held a full load of credits and worked full time.
She probably sees your "motivational talks" as just more of another sort of parental cajoling. And nobody wants to date a parental figure.
I advise freeing yourself to eventually meet someone more mature, perhaps someone who is closer to your age, and someone who is as crazy about you as you are of her. I think it's a good idea to give yourself a good year solo to concentrate on your studies and to mourn the loss of your first love (if she was that), and get to the healing stage before thinking about dating again. Take care.