It's tough man, happens to everyone.
My failsafe is one of these lines:
"Hold that thought, I need to go to the bathroom"
"excuse me for a second, something more important has come up."
"I just need to talk to my friend for a second"
Point is, when you feel that your freezing up, cut the conversation with a quick excuse, take a minute to collect your thoughts and then go back into the fight. You might DLV at this point, but it's usually better than loosing the whole battle. Having a wingman also helps a lot.
Most important, don't kill yourself over this. Shit happens, and theres nothing much you can do except lick your wounds and learn the lesson.
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http://www.solomonseagal.com
Anyone can walk into a bar and pick up a random girl.
The trick is to make the experience unforgettable to her.
A real PUA will make every women he encounters feel like she experience something once in a lifetime.