(highlight:: The grass isn't greener on the other side, it is greener where you water it.
If you want to be happy, water your own grass.)
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On top of being a maximizer (see Barry Schwartz's work), it also sounds like you're very high in attachment anxiety (see Amir Levine's Attached for more info. You're unconsciously afraid of committing to just one person romantically, as you're afraid of how much they can hurt you if they ever leave you... so you hedge your bets and make yourself feel "safe" by distancing yourself from your partner and always making sure there's some space between you. And in this case, distancing consists of focusing on your partner's flaws and fantasizing about how wonderful alternate-partners are.
Unfortunately for you and other highly avoidant people, high levels of avoidance are associated with shorter, less satisfying relationships, more conflict and fighting within relationships, and higher levels of divorce and other types of relationship problems. People who are highly avoidant are more likely to experience overall relationship problems, no matter who their partner is. (Although they work better with people are secure in attachment, as opposed to those who are highly anxious.)
I highly suggest you read the book Attached, get yourself to a therapist's office, and try to work on your attachment issues with the help and support of your partner.
Otherwise, you could very easily dump your current partner and get with a "better" one in the future... only to run into the same problems with the next girl.
The problem is not with your partner -- the problem lies within you.)
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The world is one big giant buffet. You don't have to sample every piece of pie. Find a flavor you like and eat the shit out of it.
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