How do you get over the difficulty of approaching girls?

I am introverted person and a bit shy. I dont have problems to talk with strangers for example. More likely i dont know often what to say. When someone introduces me to some girls I dont have problem talking to them or something and i feel comfortable when we are introduced and stuff. But i have problem with approaching total strangers, when you see cute girl at a bar and stuff. I feel awkward and like i am interrupting her.

How do you get over the difficulty of approaching girls?
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