I [20M] found out my girlfriend [20F] cheated on me during the Summer vacation.

Last night I checked my GF's phone and found out that her ex-boyfriend in high school had been sending her "love/caring" messages non-stop. After reading through their message history, apparently she just hooked up with him during Summer break, and a few days ago decided to break up with him because she doesn't want long-distance relationship (cross-country), and the dude was trying to get her back.

I was totally blown away by that incident, couldn't sleep well, eat well or concentrate well in class. I need some advice of how should I handle this, in the meantime I feel so sick and depressed trying to pretend I know nothing about the affair. Even though, she has been treating me really nice lately, I just don't know if I should believe in her anymore.

Some background about us:

- We did not see each other during the 4 months of Summer break. Only texting.
- I have been with this girl in my college for over a year now.
- For some stupid reasons I didn't really care too much about her for the first year, all we did were basically eating and watching movies together.
- Always strongly refuses to have sex with me.
- We had a serious talk during our anniversary 2 months ago, I promised her I would change and really take everything seriously (which I did tried my best).
- Then she began to accept my more and more. I started to have some feelings of being in a real relationship.
- She told me she was still a virgin, but one of the message I saw was the dude worrying about if she's pregnant because she hadn't have period for a long time.
- I have been behaving well in terms of my relationship with other females the whole time.
- I think the dude doesn't know my existence. Plus he doesn't have any social media accounts.

Here are some of my problems about her:
- She chose to engage in another relationship without breaking up with me first. The fact that she thinks it's "okay" to do this reallllllllly disgusts me.
- She made the decision of breaking up with the dude only 1 week ago. This one hurts me the most. I was really giving my 100% on her after our serious talk 2 months ago, and she was still treating the other guy like a boyfriend for almost 2 months.
- Lastly my main concern about why I didn't break up immediately:
- I am kind of a socially awkward kid. While my appearance often looked like the opposite, being in a college with 40k people for 3 years, I honestly don't have any close friends. I mean I can't even find someone that I can comfortably ask to have a drink with. I have been diagnosed with ADHD this May, which probably explains why my life is such a mess. Losing her is almost like losing the only "close friend" in my life. The fear of being lonely really makes me double-think every step I make about this situation.

Aaaaaaaaand I still love her deeply.

tl;dr: From GF's phone message I found that she recently ended an affair started at the beginning of May back in her hometown. I feel sad and helpless, need some advice about what to do.

So, 4 months go by (summer break) - and you don't see each other or talk during this time - just texting? Doesn't sound like much of a relationship.

Yeah, I have to admit I am a little confused/puzzled by that part of this story as well. How can you be boyfriend and girlfriend if you go literally 4 months without seeing or really talking to each other? That does not sound like a relationship to me. A friendship, maybe, but not a romantic relationship.

In all that time, did either of you make any effort to talk more and/or see each other? If so, why did it not happen? If not, why the heck not?

Bottom line, though, what she did was wrong. Even if she was feeling like she wasn't getting enough attention from you, then the right thing to do would be to talk to you about it and to break up with you if she doesn't feel there is any improvement. For her to cheat on you is bad enough. For her to continue to keep it from you and act like everything is okay makes it even worse.

Personally, I would never trust her again. If I were you, I'd break up with her because I couldn't see ever trusting her again. But, that will have to be your decision. Still, typically once a cheater always a cheater. Even if you two do get through it, how can you be sure it won't just happen again?

Good luck. Whatever you decide I wish for the best for you.