WHAT THE ****??? PLEASE HELP ME?

Me and my girlfriend were together for 1 year and 7 months. I've put up with so much in this relationship. I love her with all my heart but I have always suspected that she still has a thing for her ex. they have a baby together and he was recently granted visitations. Ever since she got pregnant with my child I feel like everything has been all about him and that she wants nothing to do with me. She always told me that she never ever wanted anything to do with him again, But now, she's over at his house almost everyday. She tells me it's because she's hanging out with his brothers girlfriend and so that her son can see his dad. But I feel like thats bullshit. I always told her how I felt about them being around eachother. She agreed. She swore to me this would never happen. But now she doest seem to care. I feel like she's trying to get even with me for the decision I made. I recently got so fed up that I broke up with her and I've begged her to come back. But all she does is tell me that "I left her while she was pregnant." "she told me to never leave her" But I don;t feel like I deserve this. I feel like this is such BS. Everyone tells me it's hormones but what if she sleeps with this or gets involved with him again? We had so much planned together now she's going back to her past. I don't understand it. This guy is a ******* loser. I've been there supporting her and the baby this whole time. I practically begged her on my knees to stop what she was doing and to take me back but she wouldn't have it. We've argued a lot since the ex came back into the picture but it finally hit the fan this time. I feel so lost, so used, so stupid. I don;t know what to do. She says she needs counseling. She was pregnant with my baby once before. She was such a *****. For some reason though she could never "remember" how mean she was to me, however she remembered everything I had said and done. She seems to have this care free attitude this time.....I feel like I don't even know her. I don't know what to do....Please help.I feel like she's trying to get even with me for leaving her. Believe it or not she's always been the biggest sweetheart I've ever met. I just don;t know where all of this is coming from.