Cheated and need advice...

I've been with a girl for about 3 months now. Due to her past experiences she claimed she had difficulty showing affection, which I was willing to be patient with her about and help her through some of the issues she was going through.

I had let her borrow my laptop for a few days and looking through it came across pictures of her at a halloween party and other pictures where she was posing with a guy, kissing and sitting on his lap etc.

This is when I thought she was spending halloween at home because she had worked late and was too tired to want to do anything.

When I confronted her about it for an explanation, she said that I was pressuring her too much and that because of it she got drunk and was just partying with her friends she had known for years.

She said she was very sorry and promised never to party with her friends, and to stop drinking.

As much as I want to believe her, I am having a hard time believing her after I found out she had been lying all along.

She does not want to talk about it, and has been lashing out on me for other minor/irrelevant things when I feel I should be the one upset.

Maybe there are some women out there that can relate and explain what is going on, and whether even to attempt at believing her and giving her another chance. Thanks.