I've had similar problems in the past and I've come up with this theory. I think what the problem is, is fear of change. You're good at making moves, acting suave around, and having solid convos with girls who are your aquaintences or friends. This is the comfort zone for many (myself included). It's easy to work here because it's framiler, it's safe. It's like walking a tight rope with a saftey net. You're confidence is high because you know if you screw up you're still good. Nothing too bad will happen all in all and knowing that allows you to game with little conciquences. This is the train of thought that your mind is going through.
Then, the change sets in. All of a sudden this girl tells you she's into you or wants to date you or something. Things have changed because this isn't just a friend or an aquaintence for you any longer, now this is a physical interest and it's staring down at you. Now your actions have conciquences in your mind and though your confiedence may not be shot, you're more self consious because you know that there's at least one set of eyes on you now. You start thinking things over that you didn't think twice about before, double checking your convos, letting stupid lines slip, maybe even act a bit more juvenile. It's like the saftey net came down and now there are conciquences to your actions even though in reality nothing has changed that much.
My suggestion is this, when you're around a girl you're just friends with before, and she tells you she likes you or is attracted to you, engrain it in your mind by anymeans possible that things are the same. Meditation, hypnosis, mind drilling whatever it takes for you to no longer think of the situation as changed should do the trick. Do this for the next few girls and you should have the problem licked. I've taken the I don't care route recently and pretended that girl who recently told me she likes me, didn't (Because it's a co worker and I'm already dating one) and it's worked out okay for the most part. I treat her just like the rest of my friends/co workers at work and she's responding to it in a way that says "he's not interested right now" kinda way...I think...darn shame cause she's pretty hot but I'm dating the second co worker in the last three months so I'm not trying to develop too much of a reputation for "getting around"
That's my take on it anyway