I'm not sure how much of what I've said won't have already been said, but I have a few points. Just so you know where I'm coming from, I'm someone who is married and has been in a successful relationship for a long time. So I'm just going to be as frank as possible and kind of lay it all out here, with no insult or offense intended to anyone:
There are a few red flags going off here.
First and foremost you ABSOLUTELY CAN NOT SPY ON HER, CHECK HER FACEBOOK, CHECK HER PHONE, OR INVADE HER PRIVACY. It doesn't matter if she is doing something to make you think she is cheating, or if it is just your paranoia, you have to either trust her or leave. You're in a toxic relationship if you're not trusting each other, but instead spying on each other. You'll drive her away by trying to control her, or at least you should and if she is drawn to an abusive relationship like that you're about to open a whole new can of worms. That aside either she is faithful and everything will be okay if you don't snoop, or she is cheating and it'll come out eventually and you're screwed anyway. Nothing can be gained by snooping.
Don't rush into commitment. A lot of people, for some reason, rush to commit ESPECIALLY when they're feeling insecure. Do not do this. If you two really are the loves of each other's lives, taking it slow won't hurt anything. Don't mix your finances, don't move in together, and for god sake don't get married. You'll have the rest of your lives to do that. In the meantime enjoy the sleepovers at each other's places, enjoy being able to surprise each other because you can't see the other's credit card bills, and don't get married because weddings put a huge strain on relationships and being married has never EVER stopped someone from being unfaithful.
Your comments about her previous relationships are troubling. Even more troubling is that you're now playing into her dating pattern, and being a controlling mistrusting bad boyfriend by checking up on her in ways you shouldn't. The best thing you can possibly do for her is break the cycle, her cycle, of dating sleazy guys. Or let me put it another way: if she does leave you, or something does come between you two, be the guy that she will always remember as the one guy who always treated her right. Look at your actions from a distance. There are plenty of things that can and will break up relationships, many of them not infidelity related, so while living in the moment also try to think about how things will be viewed through the lens of time.
All in all, while some here are taking this from a pessimistic perspective, I see reason for optimism here. You've identified key flaws in yourself and your relationship. No relationship is perfect. Because you've spotted them though, you can now work on them and improve yourself. It will be hard, relationships always are. There will be times you think you won't make it, but if you make it through they'll feel silly in hindsight. If you feel she is ready for this conversation, sit her down some time when you're both fed, relaxed, and have no interruptions and talk with her about it. For what its worth, identify your personal flaws as the lead-in. Ask her to help you work on them. If you are in a healthy relationship, she'll work with you and nurture you and help you build on that. If things aren't so good you'll probably have an epic fight but at least you'll know where things stand.
Just my 2c.