It sounds like this situation is having and effect on your other friendships as well. Since that is the case you might want to put on a brave face and do your best to grin and bear it and be around them in social situations for a time. You don't have to be around them one on one but just enough to enjoy your other friends. Your discomfort might be making all of them uncomfortable too and they take the low road of leaving you out. Not to sound old and condesending but you are probably young and they are too and life experience and time and maturity will make these situations easier.
In the mean time it is ok to feel sad and betrayed that your friend is dating someone you obviously cared for and to also feel hurt and probably envious at their happiness where you did not get it to work out. I bet they feel as awkward as you do too. Sometimes it helps just to adress that big "elephant in the room" and then go ahead and have the feeling you have. Just don't get stuck on them. Perhaps if you can spend small amounts of time with them in social settings you can slowly adjust. Who knows you might also meet someone new in that setting and be much happier.
It also might help to write a small sincere note to your friend, taking the approach that you are sorry things have been strained between you and that you miss her company and just admit that it is a strange situation for you and would she mind giving you some time to adjust. Then maybe suggest that the two of you do some girl time and just spend time together. If you value the friendship and she does too then that should be an option and I am sure she misses you too.
If the relationship with the boy wasn't a long term one then do your best to let it go and keep the good friend and take this opportunity to face something hard and uncomfortable and become the better person for it. Who knows you may end up with him as a good friend as well.
Sadly, life is full of these messy situations and the earlier you can face them and learn to handle them maturely and head on the easier it becomes and the better it will make your life in the long run.
I don't mean to trivialize your feelings or the situation.