Just found out my girlfriend of almost 4 years cheated on me and I don't know what to do

We've had some problems in the past with her being not the most faithful wanting to leave me twice before one very recent but things never got physical she never actually committed any acts before that you'd normally consider cheating but it has caused fights and her telling me one day that she wants to break everything off and then the next day she says it was a stupid infatuation. This is a lot different though.

I've noticed that the last couple weeks whenever I had said I loved her she wouldn't say it back. I asked her what was up and she said it was nothing and that she still loved me but didn't feel like being "romantic" because of school (I've been helping her go to culinery school where she met a guy she was wanting to leave me for but then changed her mind, She said the flirting was just new and she got swept up in the moment and the feelings didn't mean anything) I decided to buy that for a few days because I didn't want to start a fight before Christmas. So we spend Christmas together and exchange gifts and hers came with a sentimental letter about how much she loved me and
it made me feel a lot better for those couple of days.

Still though whenever I would say I love you before she went to school or before bed she would never say it back and I know when she does this it means something is wrong. We've been together for almost 4 years and I know when something is bothering her. So yesterday I egg her on about it I push her until she finally tells me what has been going on.

I don't know exactly when this happened but I do know occasionally she goes out to parties with her classmates she said it happened a few weeks ago though so it was most likely before Christmas happened. She said that she got drunk and passed out. Now when she said this I got upset and I knew the next words out of her mouth were that she slept with somebody else. I was assuming it was the guy from her school but it apparently wasn't. I kinda got hysterical and was really hurt and maybe I fired off to many questions and some harsh words before I could get a clear story but one moment it sounds like she passed out and she doesn't know what happen and then when I asked her sarcastically I hope the sex was worth it she said it was "Sex she didn't want" So now I don't know what to believe if she was unconscious because she said she was drunk or if she is lying to me to try to save her feelings about herself. I even asked her if this was rape that we should report it but she says she doesnt want to and that she doesn't want her parents to know...

I don't know though, the guy she slept with was apparently a childhood friend as she called him but I've never even heard of him before and she won't give me a name... I told her that I don't want to be with her anymore told her I want the money back from her tuition that I paid for but she kept begging me to forgive her and that she made a mistake.
We were gonna get engaged next year and she said that she wishes this was all a bad dream but I don't know anymore.

I didn't go to work today because i'm already not the most mentally stable person in the world and I could not sleep without thinking about her in the act betraying me... Thinking everything she said was a lie and she was just using me. But then she suddenly starts saying I love you again and my heart wants to forgive her but hate her and I don't know what to do. I've spent so much time and effort on her because I thought she was going to be the woman I married... But now she can't even give me a straight answer and that could be my fault because my emotions got the better of me and maybe I didn't let her fully explain the story...

I don't believe I should forgive her if she was just drunk... but if she was so drunk that she was passed out and got raped then it wasn't her fault right... but I don't know what to believe because she tells the story two ways... And she won't tell anyone so maybe she just made a mistake and trying to cover it up or something I don't know... At one point she said it was consensual and mistake but her first story is that she woke up somewhere that she didn't remember and started crying.

She has always been the most important person in my life these last 4 years and the fact that she has done this to me has be devastated and confused because I don't know if it was cheating or a rape or if she has been just using me... I don't even know who the guy is or if she is still talking to him or something I just need some advise that isn't bias.

I've already talked to my family and of course they're going to take my side but I'd just like fresh eyes on the situation to tell me what they think.

Thank you for reading my long post and replying in advance...