Teens wants dating

I didn't plan on talking to anyone about this, but apparently I need to. In the new year I started seeing this girl, let's her. She's the feisty, loud, has-lots-of-male-friends type, which is all great. We get on ludicrously well together, and until last week I really thought we had something special. Well, perhaps we still do. It started going wrong in the last month or so. had been blowing hot and cold, and it gave me this gut feeling that something had happened. One of her male friends had been texting her a lot (like full-blown conversations) every night, and she was starting to get even more distant. So naturally, I thought "oh great, she's cheating on me." Last Tuesday evening one of my weakest moments to date I did something I never thought I'd do: I lost my cool and read through the messages between her and this guy while she was upstairs getting a shower. Oh yes, dirty texts and picture swapping abound. I slept on it, and we talked about it the next morning. She apologised, said she doesn't fancy him doesn't even like him and was just enjoying the attention. She promised it wouldn't happen again, and that she doesn't want to lose me, etc. (We talked about my horrific behaviour too, and I'm thoroughly ashamed of myself despite her assurances that she wasn't mad about it and understands.) It's a week or so later and there are some things playing on my mind, little things about her behaviour and events that just don't add up. I won't recount them here because it'll probably make me feel worse (plus I'm bound to hit the character limit!), but suffice it to say that I'm feeling more and more like an idiot. I'm going to talk to her tonight about them all, and hopefully clear them from my mind. I'm not sure what I'm asking here. Several of you are bound to have much more experience with relationships than I do, so perhaps I just need some impartial wisdom? I really do like her, a lot actually. At the very least, she *nearly* cheated on me (I suppose it's subjective, really), so I'm nervous about trusting her again but I don't want to throw away what we have at the first hurdle.