I feel like maybe the "better" she becomes, the wider the gap between who she was and who she is...and that is what makes you consider her past as being someone else...

Contrast is what makes us notice anything. The more ideal she is to you (the more besotted you are), the less acceptable her former self becomes. In a way, you are contributing (by either your effect on her, or by your perception only) to who she is now.

I don't always make myself understood...

Trust me that the time will come when you are don't feel the same as you do now...and her past will come rushing back to her in the form of whatever you say in your anger. It takes a great deal of self-control to not do that in the heat of the moment.
It's up to you how acceptable this is to you.

Is her self-esteem such that she will forgive you for your mistakes during the tough times? The rest is easy.