A lot of guys feel this. Not a lot of guys admit it. We break it down.

I love my fiancée more then anything, but I am having one issue. I have a serious jealousy issue with her past. When I get mad at her I cant help but think of all the other guys she's been with and how she must have liked being with them more then me. I know it sounds childish and stupid, but I can't seem to make myself get past it. How do I not let this become an issue so big it ruins my upcoming marriage?

Is this childish and stupid? Yes. Then again, it's childish and stupid for me to root for my NFL team every Sunday, yell and heckle the other team's players--who I've never met, who are probably nice guys, and who'd I like just fine if they wore a different color jersey--and pump my fist when we win, and pout and bitch when we lose. Childish, stupid, and totally normal. Like your question.

Jealousy's a natural part of human life, just like cramping and indigestion. It doesn't feel good, it happens whether you like it or not, and it makes everything else uncomfortable. Most guys, to some lesser extent--okay, usually a much, much lesser extent--worry bout this sort of thing. It's normal. The trick is to keep it in perspective and not let it consume you.

This should be obvious. You know this. But we'll smack you across the face and tell you anyway: If your fiancée liked "all the other guys she's been with" more than you, she would be with all the other guys. Period.

Unless you drugged her, blackmailed her, or brainwashed her using Dr. MindBender's nano-technology--yes, that's the first and last time that we will ever make a reference from the cinematic version of GI Joe--then she chose you, willingly, and she chose you over those other schmucks.  

Here's a thought experiment. When you first hooked up with your fiancée, were you a virgin? We're guessing no. Does your fiancée have any reason to fear your feelings for ex-girlfriends? We're guessing no. Since it's dumb for her to worry, then it's equally dumb for you to worry, right?

Now we're going to make a wild inference, and if we're off the mark, feel free to chuck this and ignore it. When you say, "She must have liked being with them more than me," we're guessing, maybe, that you're worried about the sex? And, if we may be so blunt, whether she used to have a better sex life or more violent orgasms?

If this is the heart of your anxiety, two things:

1) Sex is only one component of a healthy relationship. As the years march forward and you both get older, fatter, and sprout gray hair in weird places, the sex will become increasingly less important. She loves you and wants to marry you for many, many reasons. Even if your sex is only mediocre (probably not the case), it doesn't change her passion for loving you, marrying you, committing to you, devoting the rest of her life to you. And that's the shit that matters.

We're guessing, though, that that's not really very comforting. Happily, there's the second thing:

2) Relax. The sex is probably better with you than with the other chumps. How do we know this? How can we be so certain? It comes down to this: comfort level and emotion. For women, emotion has a greater role in sex than it does for men. Emotional security matters, the love of her partner matters, all that warm-and-fuzzy stuff matters. Especially right now. You're currently in what we in the industry like to call the "Engagement Halo." (Actually, no one in the industry calls it that; we just made it up. Not bad though, right?)  Enjoy it.

So what are your concrete next steps, what's your action plan? Nothing. Unless your fiancée was a prolific hooker, prying into her past is unfair to her and unhelpful to you. (In fact, even if she was a prolific hooker, it's both unfair and unhelpful.) 

She's in love with you. She's marrying you. She's going to be having sex with you--whether you like it or not--for the rest of your life. Enjoy it. Get used to it. And don't blow it by creating an issue that doesn't exist.

Good luck.