Kwothe28

I am sorry. The burden of taking care of somebody is hard. Especially because you are a young person who should experience life and not have that kind of burden. Additionally, that person is not a family. You are not responsible for them just because you are in a relationship. As a high school junior you are barely responsible for yourself. Let alone anybody else.
As such, you are in no obligation to stay there. Until colege and otherwise. Again, you are not their family or caregiver, just a person they date. So if you dont feel love and feel burden, its best to detach right now. They will be fine and you both deserve somebody where both sides would feel the same. 

You are not a priority, move on.
I am sorry. Few years ago I was single for a while and was looking to get back into dating. It takes time to accomodate and be comfortable with yourself there. You would have to learn to "deal with blows" as they come. Especially in "hookup culture" dating sites where majority are just not looking anything serious. Most of your dates would be "one and done". This is one such thing.
She wants to explore around, and that is OK. But you are not obligated to stay. Especially when she nonchalantly said to you that she has another date. If she likes you very much she would never said that to you. Because she wouldnt want to lose you as a prospect. But like this she doesnt care. She would go on another date and if that one doesnt  be good, then she will maybe give you a chance. That is unacceptable. Always value yourself more or else others wont value you either. say to her that its fine, but you wish to not continue this further and move on to somebody else.

Until a few years I didnt even know that was a thing but it actually is lol
Anyway, he doesnt seem to reciprocitate anything at al. Doesnt flirt, doesnt wish you good morning first etc. So I think he just enjoys having a fan. 
You are pulled into an online fantasy. You both probably enjoy having someone there but there is no reality where you could be together. If he is not local, I suggest you just forget about everything and find somebody who would be there for you for real and not just typing meaningless words into computer.

I think its OK as something to unpack your thoughts. Sometimes its easier to "dump" our thoughts that way and we are better after. But I dont think you should send it at all. I often regret sending something right away. As its mostly something that was sent in "affect" and would probably not send it the day after.
As for the wife, its unclear why you did even marry somebody like that. Twice. Somebody who(from what Ive gathered from the letter) cheated on you constantly. Sleped with your friend and had a relationship with him. That is like if you see the wall in front of you. But constantly trying to run over and break it with your head. You will always get hurt badly by doing it. What she did was bad indeed. But you need to ask yourself why you were willing to put up with somebody like that and marry her twice. That is on you as well.
Also, divorce. There is no point in keeping somebody like that in your life, aside of sharing kids with her. It will be hard, sure. But in a few years, maybe you will find somebody who would love you enough not to sleep with your friends or anybody else. That should be at least minimum what you should go for if you decide on dating again.