You had to search on FACEBOOK to find the pictures! That's a lot of work to go through. Sounds like you didn't trust her BEFORE this happened. It's OKAY to admit that, you know? Sometimes those instincts we have to protect ourselves are really red flags that we already know what we're supposed to do.
She wasn't going to tell you anything about that guy she met until you confronted her. After the confrontation, she apologized to you and admitted she was doing something inappropriate. If she cared about your relationship, she would have offered up the apology and explanation before she was 'found out'.
I'm going to get some heat for this, I'm sure, but she *IS* 18, and honestly, she's probably not mature enough to handle a serious commitment like the one it sounds like you want to have with her.
I mean that, in the sense that there isn't a whole lot that you can do to *SPEED UP* the maturity process. Everyone posting on here... do me a favor, and think back to when you were 18???? She's not even old enough to drink yet, and barely legal!!!!
Yeah, yeah, I know, it sucks to hear that same "maturity speech" over and over again... but honestly, it's true. With maturity, comes an appreciation for commitment, truth, and honesty. When you're a hot little 18 year old girl, you've got men drooling all over you, right and left. You get used to the attention, and don't realize that eventually, it all goes away..... and it was never REAL in the first place. She may not figure that out for a while, and there's nothing you can do to change that.