Cherylyn

Train your daughter early because this is how you teach her responsibility.  Life is hard.  You do what you have to do in order to survive which means get help from within your own family because you cannot continue burning out at this rate.  Same with your baby as he or she grows up.  Have them pull their weight or more than their weight in order for the household to keep running.  It doesn't run by itself.  Chores and errands have to get done and you can't do everything by yourself. 
My brother was very much coddled and now it shows.  He's lazy and since he wasn't trained to maintain a household, his house is cluttered, dirty, very messy, laundry is not laundered as often (clothing / bedding / towels) as it should be and neglect is so bad that it's overwhelmingly out of control.  Front and back yards are jungles.  It's shameful; not just embarrassing. 
Train your daughter these types of responsibilities early on because someday when she's an adult, these habits will be ingrained in her and she'll know how to maintain her own household; same with your baby someday.  It's better to live a clean, organized life.  Any other way is pure chaos which leads to generation to generation mental sickness and problems. 

She could check with the host but ultimately, it's the host's choice whom she or he invites and there's no control over the host's guest list.  If your girlfriend's ex was invited to the party, would you and your girlfriend decline the invitation?  Or, attend anyway regardless? 
If your girlfriend knew her ex was at the party and deliberately refrained from telling you until later, yes I would be absolutely annoyed because withholding information is deceitful. 
If you attend the party and her ex is there, act with class.  Act natural.  You don't have to love nor hate.  Practice being polite, have good manners, remain respectful and peaceful.  You don't have to be chummy.  You can remain cool, distant yet peaceful.  There is a way to behave peacefully during uncomfortable situations. 

There's nothing wrong with you. 
Even though my story isn't the same,  I too have mourned the loss of relationships such as with my relatives.  Feelings of loss,  anger,  disappointment,  bitterness and resentment are very similar.  I have good memories of them during the best of times stemming all the way back from childhood.  We spent a lot of time together and enjoyed great times together. 
While that's all well and good,  unfortunately,  people change due to whom they meet,  marry,  some people's lives become very hard and they become harsh and cruel.  You don't know who they are anymore because they develop a Jekkyl 'n Hyde personality.  Innocence had been lost and impossible to recapture the same person they were.  As for me,  my patience wore thin and bad behavior is impossible to accept,  tolerate and ignore.
The problem is,  bad memories outweigh the good.  Nostalgic thoughts are fleeting and then it's time to grudgingly accept reality of ugly situations which went awry. 
Never say never.  Perhaps you should give yourself a break during the holiday season and start the new year anew with a clean,  fresh slate.
Give your heart a lot of time and space until bad memories eventually become a mere blur.  Time heals old wounds.  Hang in there.  Take good care of yourself. 

I don't know about him.  He may be one of those "a girl at every port" type of guys.   I'm sorry for your pain.  Learn to detach yourself from him emotionally because he already has.  He's currently in the drifting apart and fading away phase and you need to catch up and do likewise.  As hard as this is for you to do, develop the "out of sight, out of mind" mentality which will take time and a lot of space apart before he becomes a distant blur in your memory. 
Don't try harder.  There is nothing to try for and it would be an effort in futility.  The temporary relationship has since run its course.  It's over.  Remain realistic.  LDRs (long distance relationships) have a tendency to fail for obvious reasons and his heart's not into you anyway.  Again,  I'm sorry.  You were his temporary fling. 
In the future, move slower.  Don't plunge into the attachment stage so soon otherwise you'll get hurt again.  Avoid getting hurt by remaining more cautious and if you know a man won't remain within your geographical locale,  be alerted that he's not meant for you long term nor permanently.  Don't get hurt again.  Become shrewd instead.  Protect your feelings and heart from now on.  Grow smarter. 

Accept reality for what it is.  It's human nature to wish for what is not attainable.  Then think long and hard and come to the conclusion that the situation is what it is and there's nothing you can do to change it. 
Get busy,  preoccupy yourself with work, exercise,  tasks,  errands,  chores and do what you enjoy.  Reading a book or newspaper?  Decluttering and getting organized?  Take a break from the Internet.  Shut everything off.  Electronics turn into a huge time trap.  You can't get anything done.   Become industrious.  You won't have time to think of the guy from across the country because you're too busy. 
If he's on your mind too much, you might have to get drastic and cut him off completely.  Tell him the truth, tell him that you'll ghost, block and delete him for your own well being.  He'll become out of sight,  out of mind after contact had been severed. 

While I respect other people's choices,  some couples are fine with extra opposite gender people in their relationship or marriage whereas others my husband and I included are not  fine with it. 
Two is company.  Three is a crowd.  ☹️
Your bf is deceitful.  There is something shady about him.  He lacks integrity.  He's a liar.  Once a liar, always a liar.  You can never trust him. 
Either she goes or you go.  Your bf can't have it both ways. 
As long as you're with your bf, these wounds will continue to fester.  He won't change.  He is who he is.  Either accept him the way he is incurable defects, warts, baggage and all or go through that door and don't come back.
He does not sound like a keeper.  Let him have his female friend all he wants without you in the picture.  Treat yourself better than that.  You deserve to be treated with respect and class.  Any other way is intolerable AND UNACCEPTABLE. 

Note that he manipulates the conversation whenever you stay on one topic at a time.  He's gaslighting you.  Google "gaslighting."  One of the gaslighting tricks and techniques is to deflect, change the subject and force you to change your perception of the facts.  It's psychological warfare at its finest. 
Both of you are incompatible.  It's time to go your separate ways permanently.  Don't waste anymore years on him.