Hey everyone.
I will lay my situation out real quick in point form.Please note, that any of my gf's communications and actions with the other guy were unknown to me until a few days ago.
My gf and I got together 1.5 years ago. Things were great until about 6 months ago.
At that point I started getting really busy with law school and neglected her.
During that time, for some stupid reason, I was sexting another girl (someone who I had never met up with, or did anything physically with).
My gf forgave me for it and we moved on.
This August (2014), we went on a vacation and it was sort of rocky because she said I was still being distant.
Fast forward to September, she tells me she needs more attention and affection. So we have a good chat. I tell her about some past abuse issues and death of loves ones which cause me to put up walls and not fully put myself out there for someone. But that I wanted us to work on that.
The first weekend of October we broke up for two days because of a fight. That weekend she went to a party, met a guy and they kissed.
The following day her and I got back together.
My gf and that guy kept chatting via text, Facebook and snapchat (I didn't know about it).
For the whole month of October she lied to me a few times and said she couldn't see me because of work. But it was so she could hang out in a group setting with him and other friends. She took him coffee to work and hid that from me as well.
November 6th, I thought things were going a bit better. Then we had a little argument, nothing major. And she said she couldn't do it anymore. She was crying and said she couldn't do it anymore. She left my place and said we couldn't be together.
The next day she agreed to meet with me for a few moments so I could drop off my journal that explained how I fully felt. It said how I was sorry for the past, that I only want a future with her. And it explained why I am distant and have trouble becoming attached. But I made it clear that I would do anything to work on that.
So, after dropping that off I left. I met up with my gf's mother for a coffee where I cried to her about how sad I was and that I wanted her back.
That same day my gf (although ex of 2 days) was continuing to message that guy. She had a small party at her house and invited him. They all went to the bar and him and her were making out. Just the two of them went back to her place and she gave him a blowjob. She claims she was extremely intoxicated and doesn't fully remember, but does remember doing it.
The following morning after that guy left her place, she texted me and said she needed to see me. I came over and she said she wanted me back and that she was sorry for breaking up with me. At that very moment she said she had something to tell me. She told me she was chatting with some guy and that he wanted to hangout with her along. She said she refused him and there was nothing more to the story.
So, her and I continue on with our relationship for 3 weeks and I am 100% in love, completely happy. Then, I get an anonymous message that says "she isn't telling you the whole truth." I call her out on it, she gets really nervous and says "I gave Brad a blowjob."
I was so hurt and shocked that I left. We talked the next day and I said I still wanted to be with her. But I was hurt, and I thought there was more to the story. So a few days go by and I tell her that I have to contact him to find out the whole story. Thats when she says ok....and proceeds to tell me about the previous 6 weeks (which I outlined above).
She tells me it was all a mistake. That she never wanted to be with him that night. She says she kept chatting with him because he was filling an emotional void that was between us. And she says that she didn't want the blowjob to happen. It was just a combination of her being drunk, vulnerable and still upset about me. And she expects me to just move on.
We have been to two counselling sessions so far. But, I can't shake it out of my mind. Every time I see her, each time she tries to kiss me, I picture her kissing him or her giving him a blowjob.
At this very moment she is laying on my bed and seems relatively content. I am an emotional wreck.
Can I get over this? Why would she listen to my deepest stories, know my full past and how I feel, then get in bed with him the same night? Why would she kiss him, then be with me for the next 6 weeks only to break up with me one night, only to call me back after she spent a night with him?

Background info:
I am 32, divorced with a good job and am in law school.
She is 23, mediocre job and I am her first boyfriend.
Despite the age difference I am completely in love with her and want to make this work. I just don't know how to get this sick, disgusting feeling out of my heart and mind each time I think about it.