NRE And Quantifying Love
I like what RedPepper said about the "New Toy". I'd have to agree that what you're going through sounds a lot like NRE brain chemical overload. These are real drugs, and should be respected in that light. They make small things seem wondrous and cloud the issues that may later become more clear. They can make sex or anything else feel better, and obscure from you things that should feel worse.
My advice would be to make the mental association when you are with your new lover to the fact that your being with them is a testament to the love and gift your primary partner is giving.
I have seen friends throw good, strong, loving relationships away because they get caught up in NRE, figure that they must have made the *wrong* choice in their current relationship, then later get wildly depressed because they realize what they miss, after the NRE wears off.