I'm so sorry you're having to deal with this.
This may not be the advice you want to hear, but I think you need to look for a new church and make a new circle of friends. Without getting into the religious side of things on these boards, the whole idea of a group of friends who are close through church should include some accountabliity among the group. I find it hard to believe that nobody in the group picked up on what was going on, or that there was no secret keeping from you at some point either before or after the break up. With friends like that, who needs enemies?
Also, you say you are both young. To rollercoaster from asking you to marry him to breaking up in two months shows an incredible lack of maturity on your ex's part. It implies he did the asking while he had doubts. Perhaps he was caught up in the idea of marriage, perhaps he felt pressure from you or the group to make things official, or perhaps he's just not as nice as you think he is.
Whatever the reason, he has acted like a child, changing his mind about what flavor of ice cream he wants, not like a mature man ready to make a lifetime commitment. I don't think you need to spend a lot of angst on what is wrong with you.
And while I'm on that particular soapbox, your "best friend" is just trying to make herself feel better here. A good person who realizes that she has feelings for someone else's fiance removes herself from the situation, period. She doesn't steal the fiance and then hope an apology makes everything all better. Maybe she is sorry that you got hurt while she was getting exactly what she wanted, but that didn't stop her from taking it.
So, I agree with others that you need to move on. From the situation, from this group, if you're young enough to pick up and go, then maybe even from wherever you live. Give yourself a fresh start and a chance to heal and grow into whatever good things life has in store for you.