My First Breakup

I've been officially inducted into the heartbreak club. Here's the slightly confusing, very sad (for me), rundown. (TLDR at bottom) My (now ex) girlfriend and I have been talking and being "unlabeled" for about 4 years, on and off, officially dating for over 2 of those years. Our relationship had gotten very serious; we discussed marriage on several occasions, our hopes and wants for the future, and were close to moving to be able to see each other more (it was long distance). Fast forward to the end of January, she was sent to a therapeutic wilderness program to help her battle with severe depression. She was there for about 3 months with no contact to the outside world including myself. Following the wilderness program, she moved to a different, transitional program where she could learn basically how to be an adult. She has roommates and neighbors her age. This program is still currently going on and will for the next year. Due to the first program and her busy schedule, we did not get back in contact (other than the occasional letter) until the very end of April. Everything was fine and dandy, and then fast forward to this past week. She said that she needed to talk to me about something, and preferred to do it over the phone. I said okay, so we planned to talk. I assumed it would be something serious but did not think about separation. Days go by, of her flaking out on me and giving me different reasons every time. At this point it had been 3 or 4 days of me knowing there was something that needed to be talked about, but I had no idea what. After some time, I decided I would rip off the band-aid for my anxietys sake. I asked her if she was planning to break up with me. She said yes. We still planned to call and talk about the circumstances surrounding it. Again, a day or two goes by where she flakes out. We call. She tells me that when she was in the wilderness program she fell out of love with me, and upon being at her new program, fell IN love with a boy. I was heartbroken. This whole situation has been very confusing, from the lack of communication, to the professing your love to someone mere weeks after knowing them, to throwing away a 2+ year relationship for it, to her sending me letters from her program about how she wanted to mend out relationship, the list really goes on. I have never experienced a breakup before, and this is very new to me. As upset as i am about the circumstances, I really want nothing more than for her to come back. I have cut contact with her for my sake aside from her phone number in case she really needs something (and I think part of me wants her to tell me how bad she messed up and that she wants to be with me). I guess, as a newbie, does anyone have some advice or anything? TDLR: my girlfriend of multiple years fell out of love with me and fell in love with someone else. This is my first breakup and have no idea what I'm doing.