My girlfriend talked dirty with her ex.

Okay, so, I'm known for never really going steady with a woman, I was always the guy that went to parties and hooked up with one night stands and all that ****. I did have some girlfriends every now and then, but they never lasted more then 3 months and most of the time, I cheated on them. I was 14 when I lost my virginity and I'm almost 20 now. I'm in a good, stable relationship where I've been completely faithful and honest with her, because I really do love her. She had one boyfriend before me, they were both virgins and lost it to each other and both were first loves for each other. He treated her like complete c**p though, cheating on her, throwing **** at her, yelling at her all the time, and by the end of the 2 years they were together, she told me she was disgusted with him and cringed at the thought of having sex with him. So as soon as they broke up, we got together. Yeah for a little while, I was thinking it could of been a rebound, but it's still going. We're only at 2 months, and she has already been unfaithful to me, despite how perfect I've been to her. What happened was, we were having a argument and I wanted to take the day away from her, cause we been practically living together. I allowed her and her ex to talk and be friends, cause I understand what kind of impact he made to her life regardless, so I trusted her to just be friends with him. Well I found out that night we were arguing, that her and her ex were talking about "What if they got back together" and they were talking dirty to each other. I was furious, I'm Irish and I do have anger problems, and I was seeing red. I'd never hit a woman, but I do take it out by beating someone else up that pisses me off. Anyway, I drove my *** over there so fast, cause I wanted to read what they were saying to each other, I had to, and it was some pretty intense dirty talk, but then she told him "That she feels bad for doing this to me, Matt(Me) doesn't deserve this, and I wanna be with him" so that all ended. I told her it was him or me, she had to either stop all communication with him or I'm gone. She chose me. We had sex and I left to go back to hanging with a friend of mine that was leaving for Afghanistan. The next day I asked her, "If you two hung out, would you have sex?" She told me, yes, they would, even though she's with me. She said "Oh it's just because how we are." I forgot to mention that she also told him that she loved him very much, but that she loved me more. So she still has feelings for him, and if they hung out, they would ****. Now, I want to give her another chance, cause I don't wanna let a thing like this destroy us. But now that you know what has happened, I'll get to my issue. It was the day after all this happened, we had sex again, no real problems there. But 30 minutes later, we were fooling around again, and she went to give me head, and I was really turned on and wanted it, but I just lost my erection. We tried again late night, she went to give me head, I really wanted it, I lost it. That one night she was having dirty talk with her ex, she was talking about giving him head, so I think I got that floating around in my head too, and just everything that she said to him. I'm typing this the morning after I couldn't keep it up, what should I do? Should I give it a few days and just try to clear my head? If it still doesn't work, are we just finished with the relationship? Is there anything I can do to help me rid of this problem? If anyone has any tips or advice on this WHOLE situation, please tell me.