Am I too gullible? What should I do?

I've been with my girlfriend for almost a year now. It's my 1st serious relationship. I have had one girlfriend before her but it wasn't serious. This is the only time I have ever been in love. I can't see myself being with anyone else, but I can't be sure that we will spend the rest of our lives together. She is one year older than me, and I was admittedly a late bloomer, but I have certain morales. I always thought that whoever I would end up with probably would have experienced more in their past than me, but I never would have thought I could accept someone with the past that she has, and I'm still unsure that I can. I am 22. I have had 3 sexual partners and 2 girlfriends. I lost my virginity in a one night stand, which I am very ashamed of. Within 2 weeks of going out with my girlfriend, we had sex. I am not the type to go move that quickly in a relationship, but I never felt a connection like I did with her, and I thought that she must feel the same way, so it felt right. I didn't have any condoms, as I didn't plan on having sex with her so soon, but she told me that she was on the pill and i didn't need one. Stupidly I didn't think about why she was on the pill, as obviously she didn't start using it from the moment she met me. But she really didn't seem like that sort of girl. Early enough into our relationship she asked me about my sexual history. I told her everything, admitting that I hadn't been with many people, because I haven't had many girlfriends. I also told her how I lost my virginity and that I was ashamed. She was really suprised that I hadn't been with more people. Foolishly I didn't ask her about her history. From up to that point I can take full responsability for our problems, because I didn't have the courage to ask about her past. I knew that she was in a serious relationship with a guy I know. She told me she lived in New York for 2 years, where she got engaged, but the guy was abusive so she came back home. She told me that after that she had gave up on guys and before me she never thought she would be with another guy. Just less than a month into our relationship she had left her phone in my car, and I looked at her messages. There were a lot of messages to other guys, one in particular who the messages were very affectionate towards. But she did tell me at the start of our relationship that she was friends with a lot of guys and didn't want me getting jealous. I didn't assume anything bad, and just felt guilty for invading her privacy. However, about a month into our relationship I heard a rumour that she had cheated on her 1st boyfriend. I didn't believe it as she didn't seem like the type to cheat and the guy she supposedly slept with was a low life drug user. But she later told me that she had done a lot of weed before and had experimented with e-tabs and coke. I asked her if she had cheated before and she confessed that she did, but only because he cheated 1st. I was still disgusted, as I hate cheating. After a pregnancy scare with us, she confessed that when she was with her ex-fiance she had an abortion, but said it was his fault. After explaining it to me she did admit real guilt towards it, but I really thought it was unfair when confessing it to make out as if she was completely innocent. Round that time I asked her about her sexual history, as it was playing on my mind, but she wouldn't tell me. I let it rest for a while, but eventually plucked up the courage to demand she tell me, as she had no problem asking me so early in our relationship. She told me 5, and I was so relieved, but so amazingly stupid for believing her. After that, one night after using her laptop, she had left her email password written on her laptop. I felt really guilty looking, but I couldn't help myself and looked, thinking that I wouldn't see anything bad, and that would clear all of my worries. I was so wrong. What I read turned my stomach. She had messages from her ex fiance, asking why she was telling people that he beat her, and they both know the truth about what happened. There was also a lot of accusations of her cheating. He also talked about a lot of their drug use, and in detail discussion of threesomes they had, involving another guy. There was also a lot of messages from guys who she was clearly intimate with between the time she left her ex and meeting me. There were also messages from guys she used to see, but that she had recieved whilst she was with me. Also there was a message in her sent box that was sent on the day of our 1st date. It was sent to a guy from Dublin, and she said that she was staying in Dublin 3 days later and she really wanted to meet up with him because she was dying to see him. She had also been to Dublin with a friend whilst we were in a relationship, and an email dated for a few days after that from the guy said that he was sorry but he just wanted to be friends, even though she wanted more. I told her that I was sorry that I had read her messages and I needed to see her. I said I had messed up big time and I could see her so I could explain myself. When I got to her place I apologised for invading her privacy, and tried to bring up what I saw. She tried to blow me off, saying that I was at her house to explain what I did wrong. That moment was the first time I ever stood up to her. I said that what I did doesn't excuse the things I saw, and told her to explain everything to me. She said that everything that her ex said was a lie, and that looking back on their relationship that he clearly was mentally unstable. She had bills from the hospital to prove that she was beaten, and very badly, and said that she never had a threesome, even thought he would invite girls to their place to have threesomes, and she would not get involved, so her ex would end up having sex with another woman. She also said she was never unfaithful, and couldn't be even if she wanted to because she was too afraid of him. But she did confess that she had been with more than 5 men. She admitted to sleeping with 12, 8 of which were in the time between her ex and me, which was less than a year, which completely makes a mockery of everything she had told me. Before these confessions she also used to talk badly of girls who sleep around. I also found out that her ex fiance was 10 years older than her, which was ironic and he belittled me when she found out that my ex was 3 years younger than me. I let her know that she was a hypocrite. If I had of known these things at the start of our relationship we would not be together still. I also brought up the messages from the guy in Dublin, and she assured me that nothing was going on when she was with me and she didn't know why he said the things he said. But a while after this I found her camera which had pictures of her with the guy from Dublin, and they were dated from the day that she was in Dublin whilst in a relationship with me. She said that the dates on the camera were wrong, and its just a coincidence that they happen to be on that date, because they were taken before she met me. There were also pictures of her coming home wearing certain clothes and I remember her wearing the same clothes after coming home from Dublin when she was with me. I've brought up my doubts many times to her, but she says that she is 100% honest with me now, but am I too gullible, because looking back it all seems like too much to believe. What do you al think? If I found out that she cheated on me in any way I know I'd leave her, and I think she knows that, so if she did I know that she would deny it, because I know now that she loves me and would never betray me. But I don't want to be in a relationship based on lies.