Unable to smile/look at girls

I was wondering if anyone has this problem. I am physically unable to get myself to show interest in girls in public, for example:

- I can't bring myself to genuinely smile at someone, without make a half fake-smile which looks forced and awkward
- My default facial expression is quite serious, but I'm pretty friendly on the inside
- I can't bring myself to look at a girl I'm interested in to show my interest, because I'm paranoid that other people (i.e. in the station) will notice and think I'm pathetic.

How do I get out of this