<human>: Condense the text into a few key points

I've been dating this girl for a little over 2 years now, and we're relatively young, but it's a pretty stable relationship, and normally I'm happy with it. She cheated on me a couple weeks ago, and ruined the best friendship I've ever had. I'm working on forgiving her and being able to trust her again (Also, by cheated I mean she let him kiss her). Just last night, she was having a movienight with her friends, that was perfectly fine with me. They were gonna stay up late and watch movies. I didn't care, I wanted her to have some fun with her friends, she hasn't been around them in awhile. I went to bed around midnight, and she was acting kind of funny right before I said I was going to bed, then two hours later (about 2am) she posts to facebook asking people to snapchat her (I told her not to get Snapchat, that I don't trust her sending pictures back and forth with people right now that I can't check up on or see to make sure they aren't anything bad.) She's asleep right now still, and I'm driving myself slowly insane thinking what she could have sent, why she told me she didn't have it, etc. 

I hate that she keeps lieing to me, and I really really want to take her phone and throw it against the wall 90% of the time. We didn't start having these problems until she got texting and a smartphone. I keep thinking if I eliminate the problem it'll solve somethings (or at least help) but I know it'd only serve to hurt our relationship in the end.
<bot>: My girlfriend cheated, has the means to do it again, lied to me about it, I'm angry and probably overreacting a bit, and I don't know how to handle myself.