The Smell of Desperation...

You ever notice how when you're in a relationship, women tend to be more attracted to you, even though they don't know you're taken? But when you're single, they stay away like someone sprayed you with a can of "OFF! Celibacy Edition"? Or like your girlfriend sprayed you in the neck with her pheromones to mark her territory, but it seems to have the opposite of the desired effect? Well, I'm going to tell you why that is, dear reader. And hopefully open your eyes to the truth, despite the fact that there's nothing you can do to use it to your advantage whether you know it or not.
The reason for this sudden change in behavior from women isn't because they see you differently, it's because you are different. The truth is, when you're in a relationship (a moderately happy one, at least) you're a different person. When your lady makes you happy, your behavior changes subtly into the behavior of a more attractive person. You act differently around women because you're no longer wondering in the back of your mind if she's hitting on you, or if you should try and get her number, or if she has a boyfriend. You stand up taller, you're happier because you're not going back to your lonely man cave alone to wonder if you missed out on asking that pretty cashier lady out, or maybe you'll ask her out tomorrow. But she probably has a boyfriend. Whatever. She wasn't that cute anyway.
You may have heard this once or twice, but women love confidence. Not cockiness, but confidence. Sure there are some special cases where they can't get enough of your self-loathing and doubt, but the safe bet is usually in the opposite direction.
The reality of it is, you're never more confident than when you already have a good woman in your corner. You go from this:

to this:

...overnight. 

Now I know this sounds like a catch 22. You need a fish to catch a fish and you can't catch any fish because you have no fish.
Fish.
But it only is if you let it be. If you're having problems finding a girl because of confidence, that's an in-house issue. You need to embrace the good parts about yourself and go ahead and show them off. Build your own confidence.
And if you finally found a girl who can be there for you when you're at your best and your worst, then you don't have any reason to even worry about those women out there that only notice you at your best. And if she's the reason that you're at your best, then she's probably worth keeping around.