Unsure how to go on

I have been dating my current girlfriend for 4 years now. When we first started dating things were great we would text message each other dozens of times a day. I would drive out to her work just so that I could see her before she went home for the night. I worked during the day and she worked at night. We went out at least once a week either to the beach movies or a club. At one point she had gotten drunk and made out with another guy. She came right out and told me about it. I forgave her because I loved her very much. After dating for 2 years we decided to move in together. When we first started living together things were great. She would help with everything around the house and we made dinner together and really enjoyed each others company. We still went out at least once a week to keep things fresh. After about 6 months work was really draggin me down so I didn't want to go out that often. She would always get mad at me and say that I was pawning her off on her friends. I didn't want to keep her from going out and having a good time because I was tired. Just before I got orders to florida we had a falling out. She left me and moved out of the house. Only a week after we broke up she started dating another guy. I was heart broken. I didn't want to stay down in the dumps and mope because it hurt when I didn't have anything to do and I just sat there thinking about her. I started going out alot with my friends and I was starting to get my life back together. I kept in contact with her because even though we had broken up I still loved her. About 3 months after we broke up she came back to me because she said that she realized that she made a stupid choice and that she really loved me and she couldn't stand not being with me. I took her back and we worked our problems out. She moved down to florida with me and things were going great again. Just a few weeks ago she started acting strange and seemed like she was really being distant with me. I came to find out that she was talking with her boyfriend before me and that she was telling him that she loved him. I also found out that she had been hanging out with another guy and that she had been making out with him in her car after work. I confronted her about all this and at first she acted like she didn't know what I was talking about. After I told her that I knew everything she started spilling the beans. She started crying and saying that she didn't want me to leave her and that she loved me very much. At that point I showed her the engagement ring that I bought for her and ask how could I possible ask her to marry me if I am always looking over my shoulder and wondering if and when this will happen again. I remained calm during the whole conversation because yelling would have gotten us no where. I asked her what we needed to do to fix this she told me that she would do anything that I told her to fix our relationship. I love her so much that its really hard for me to think about letting her go. I told her that I would not tell her what she needed to do to fix this because if I did that I wouldn't mean anything. I told her that if she really wanted to be with me that she would have to figure out a way herself on how to fix this problem. She only said never again. I took her back but there are parts of me that want this and other parts of me that believe I should deal with the months of pain instead of many more years of doubt and untrust.

On a side note we had sex a few nights after this happened. I didn't want it to happen because I seemed like it would paint a picture of everything being okay again. As of now I am upset that it didn't happen.

As much as I want to be with this girl I don't know how to go on.

Please if someone could give me some sound addvice about this other than the one liner "drop her" I would greatly appriciate it.