If you want better, hotter sex in (or back into) your relationship, then learn how to have better, hotter sex
I guarantee that great sex has little to do with the size of your dick (and therefore, little to do with the largeness of the next guy's.)
NRE makes sex better, no doubt. You ought to be reaping the benefits of spillover NRE. If you're not, that'd be a big indicator to me that there are other issues going on.