I don't know what to do and I need help!?

So me and my gf have been dating for about 2 years... Me and her recently got into a "big" argument according to her. Let me give you some background info first. We have known eachother for 6 years and we been dating for 2. She has cheate on me 2 times and I took her back both times... She fcked a guy while we were dating -_- and she claims she is sorry for it. She has done a lot of stupid thing that make me mad, like sneaking out of her house at 1am to go to a guys house to drink at a party -_- Im making it seem like she is a bad person, but I have done things to that were stupid. After the first time she cheated on me I made out with a girl and fcked plenty of girls while we were dating... But after al that bad stuff happened we had a great time together after we put our past behind us. I love her with all my heart and I never wanted to loose her.. So a couple days ago I didn't get to hear from her for about 4 days and I got worried and looked on her instagram and saw something that