OP:
There are a couple of quick points I think may help you as they have helped me, but before that a little background so you know I'm not just talking about something I have no experience of.
I messed around a tonne when I was a kid/teen, I did a lot of stuff I shouldn't have, and have had to seek God to put it all right before him. It was tough, but I've done it and I'm glad I have. I still remember having to tell my girlfriend about past "dalliances", and it was crushing having to tell her. There were also parts of her past I struggled to hear too, so I know from both sides of the coin the problem you're having.
My first point would be that although it's difficult to get over these sorts of hurts when you find something like this out, it's not impossible. Remember, she didn't do any of the sinful stuff
to you, she did them to God. If Jesus has forgiven her, surely you can too. Remember, Jesus died for the sins she has been humble and brave enough to admit to you (which is, in and of itself a great sign of her respect for you as it would have been tough for her), so you should accept that and pray you can move past it. If you can't then maybe you're putting yourself above Jesus.
Secondly, putting God in the place of highest esteem in your relationship will be a great idea for you. If you seek God out in all of your problems you will find it easier to accept and move past what is a raw emotion for you at the moment. As you rightly pointed out - her past is currently very new to you, and hurts like that can take time to heal, but they will do and when they do she'll still be there, and God will still be calling you to be the man she needs. You need to love her in a way that transforms her, and you can't do that until you've worked past the past. That make sense?
As a side note, I don't know what age you are, but make sure you're old enough to consider this relationship leading to marriage. Don't date at a time where marriage isn't an option, or won't be an option in a realistic amount of time.
As for the point Persecuted raised I would say this - don't let what culture has to say on issues affect the way you see things as a Christian. There's always the danger of saying "culture has evolved beyond what the Bible comprehends as it's such an old book", but to do so is to call God's word fallible, which it is not. If you consider the divorce before you marry or even date (as you referred to the recent Nickelback divorce) then you're already entering into a marriage expecting it to fail. That's not the kind of faith God calls us to as men. Further reading on this point would be 1Peter 3, 1Timothy 3 & Ephesians 5.
Furthermore, you mentioned the feeling of going into a marriage thinking she has one over on you. That's a really, really dangerous way to think. I would say any man who thinks that way of his girlfriend should not consider marriage because he is still a boy. This "one-upmanship" attitude is childish, and 1Corinthians 13:11 says
"When I was a child, I spoke like a child, I thought like a child, I reasoned like a child. When I became a man, I gave up childish ways." So any man who thinks this way should too give up his childish ways and become a man.
I, of course, don't say that to judge or condemn, that's not my place. I just want to make sure no one is making mistakes that are avoidable.
Ninja250, this applies to your situation I feel: I understand the past can creep into the future, that is always the case. Fearing you're not what your girlfriend wants is a very valid fear that a lot of Christian guys struggle with. I would say that if she has picked you to date, and picked you to marry, then you must be doing something right.
But at all times, as Proverbs 4:23 says – “Guard your heart above all else,
for it determines the course of your life.â€
So guard your heart.
Also, the Bible calls us to have our spouses as our standard of beauty. If anyone should ask us to picture our perfect woman we should picture our wives. So if you're with someone skinny you like skinny. If you're with a blonde you like blondes, and so on. This means, when dating, you need to find the total attraction (attraction of physical, mental, spiritual and emotional), otherwise she can't be your standard of beauty.
I apologise for the long post, I tend to ramble. I'm also sorry if some of this is not quite relevant to what people have said, I'm tired and it's been a long day so I may have misread some stuff