why am I so scared to talk to a girl?

Your problem is you are too shy and have also not yet realized you have nothing to lose. I mean, if you talk to some girl and she doesn't like it, what happens? Is she going to bite your head off like a praying mantis or eat you like a female spider does to the male she just mated with? Obviously not. So, first, you need to get over being scared. Let's try something low key and low risk. Go out to a mall and walk around. Say "hi" or "hello" to every lady that you see her face. Most will reciprocate. Then just walk on. Keep doing this until you can do it for a full hour and not feel self-conscious. Once you've gotten past the fear of doing a cold open on a lady then being able to talk to them is no longer a problem.

you are just trying to protect yourself from the pain of rejection

Once you confront this fear you will no longer be driven by it

Life is seldom as hard as we choose to make it

You have nothing to loose and everything to gain just go for it.

This is all about changing the odds into your favour. At present the odds are low because you never ask a girl out but once you start asking girls the odds swing very quickly in your favour.

Just get past yourself you will do fine

You are araid of rejection