"What I really want is to love my fiancee the way that I know that I can."
If that's what you really want, then do it. It really is that simple, if you let it be so.
I've had to tangle with reasons and feelings myself on occasion. One of the best ways I found to change my own feelings was to do the things that I would do if I really felt some way, and soon I started feeling that way.
It's kinda strange, but it works.
In other words, if I want to make someone else the most important part of my life, I start acting like I would if that were the truth, and suddenly I notice that, hey, this really is the most important person in my life.
Actually, this one I know from personal experience. Do yourself a favor and don't have to learn that one the hard way -- I've already learned that, and you might as well benefit from my experience.
If you want your fiance to be that most important person in your life, then treat him that way.
If you want your ex to be unimportant or less important, then treat him that way. Stop having these little chat room liaisons (if you're keeping them secret from your fiance, that is). Stop feeling like your ex is the most special man you'll ever meet. You're the one making yourself feel that way. And I know this might seem like a difficult one to accomplish, and you might have to work at it -- a lot. When you notice that you're feeling that way, find something else to do to put your attention on. Work on feeling how you feel about your fiance instead. Yes, you won't master this over night. It took me longer than I wish to confess at the moment to regain control over my feelings (which were on some bizarre autopilot for the longest time...), so don't be surprised if you falter a lot. Maybe you'll master it overnight, maybe not. Just keep at it. You'll get better with practice.
If you want the fact that you have been harboring these feelings for your ex to be unimportant, then prove it -- tell your fiance about them (and your chat room stuff, and anything else that you're actively keeping a secret from him). There's not really anything wrong with wondering -- premarital jitters are common. Don't you realize yet that, by having these little secrets from your fiance, you're creating the exact situation you say you want to avoid by not telling him? I wouldn't suggest making a big production out of it, as if this is some confessional or something, because it needn't be. If you have really chosen this man to be yours, then treat your recent actions and feelings about your ex as an afterthought, something minor, because, really, that's all they are. But don't hide them, either, because all you're doing now is creating trust issues of your own!!!
(For me, I discovered that, the moment I told my wife about things like that, the feelings suddenly stopped being important at all. Come to find out the "hiddenness" was a big reason that fueled the feelings.)
On the other hand, if you want to go back to your ex, then just keep doing what you're doing, because that's where you're headed. This isn't necessarily a bad thing, either, by the way. But is this what you really want? If it is, then go for it.
Decide what you want to do and then start doing the things that would demonstrate your decision. Knock off doing things which demonstrate the opposite.
Another important point: Don't beat yourself up over it. Any guilty feelings you might have over anything that has occurred up to now (OR IN THE FUTURE) will only give you reasons to punish yourself or to take away whatever your real choice is from yourself. THIS IS A TOUGH ONE. I know from personal experience, and I've watched other people do this, too. This one is so hard to give up. "But I don't deserve..." As long as you give yourself reasons to believe that, then it'll be true, and you'll make things happen that way. "But I should have (or shouldn't have)..." Yeah, so? Well, now you know. Be willing to let yourself make a mistake or two (or thousands...) and then move on. The black and white stuff is easy. The gray area is where we test ourselves and figure things out.
Stop believing in your reasons, and the feelings will get back under your control. "But he's my soul mate..." Yeah, so? Sometimes it takes a lifetime or two (or thousands...) of missing that chance with a perfect partner to finally get it through our thick male skulls that we need to step up to the plate and get past those trust issues so that we can have that perfect mate. That's no reason why you shouldn't be happy now, and if he really is your soul mate, then when he finally comes to his senses, he will agree with me on that one.
When you manage to pull all this off, then I promise you will no longer have any "self esteem issues".
=-John-=