I understand, first of all, you have to understand that you're not alone. Many other guys, and women have had the same thing happen to them.
During the early years of high school, my girlfriend cheated on me (sexually). Then again during college, my girlfriend cheated on me. Then again during Law School, my girlfriend cheated on me with one of my classmates!
At first I thought, maybe i'm just a boring person. Then I started to think that I needed to work out a little bit more, work on my interests, or find new friends. I then started to doubt the women i'd meet, and always look out for if they did anything that would leave open the possibility of them cheating. But I realized that this wasn't the way I wanted to live.
Given, out of the 7 people that i've dated, 3 have cheated on me. That's nearly half! But I know that something went wrong in these relationships. I know that looking back on it, If I could be with that person today, I wouldn't want to; not because they cheated, but because we really aren't compatible or satisfy eachother fully.
Think about that. If you could, would you want to be back with your ex? Didn't think so. Now, think about how wonderful the person you have next to you is. Think how great she is, and how luck you are to have been "freed" from a life with someone you didn't connect with, and given the chance to meet and fall in love with someone so wonderful; who completes you, and whom you can complete fully.
Do you really want to let your past, the history of someone who was not worth it, ruin what the two of you have now? Think of it this way; is it really fair for the women you love now, to suffer for the wrong-doings of someone else?
I'm a safe driver, and I try to explain to my insurance company that the premium they charge me is unfair, because I never drive over 30, "on the highway!", and never over 15, local. They however feel that men in the past have had more accidents than women, and have a higher risk, so that I must pay this unfair price which I don't deserve.
This is a simplification of how "She", (you didn't use her name), must feel. She won't cheat on you, she's not even that type of person, but you treat her like she is. This is insulting to her, you are suggesting that she doesn't respect you, love you, or have any principles. You take away the most powerful force keeping couples honest; respect.
Think about double-jeapardy. If you've been convicted of murder, served your punishment, and then had the chance to kill that person, what is stopping you from doing it? Nothing! The same goes for respect. If you treat "her" like she has cheated on you, or like she is one to cheat on you, then you have broken your respect for her, and unlimately, she won't feel like she's "loosing" any respect from you by cheating.
I'm not saying that she'll ever cheat, i'm just trying to show you how she feels. It's actually very depressing, being accussed or punished for something you're not guilty of. Treating her like she might cheat; always being cautious of her, jealous of her, is like you're punishing her, for something she'll never do!
The easiest way to get over it, is to take an active approach. Listen to what i'm about to tell you, and just do it. You'll be surprised how easy it is to set things right, and keep them that way.
Go to her, right after you're done reading this message, and tell her something along the lines of, "I'm sorry i've been acting jealous. You really don't deserve that from me. I love you, and I know you love me. I trust you blindly with my life. I know how honest you are, and that you're just not that kind of person. I promise, to never let wrongful jealousy come over our relationship; only if you promise to slap me if I ever forget to keep my promise."
It's that simple. By doing this, you just openly prevent yourself from acting jealous towards her again. You will have this open and public promise in the back of your mind. Everytime you start to get jealous, you will remember the promise you made, and you'll start to feel bad and correct your thoughts. Very quickly, and quite naturally, this open promise you will make to her will not only immediately take jealousy out of your relationship, but permanantly remove it from your thoughts in a relatively short amount of time.
I hope this was helpful. Don't waste an other minute, and go make her that promise. You owe her, the woman of your dreams, that much to make things better between the two of you! Best of Luck with your relationship!