It's okay to miss oppertunities. Other oppertunities will come along.

If this is about something you do want to do in the future, you can start working on it now, preparing for the next oppertunity. Just don't put too much pressure on yourself!

You can work on it in two ways: practice your English speaking more with other people, people you trust who won't laugh at you.
And I would also suggest, next time you get an oppertunity you don't want to miss but you feel you might avoid - sit down and write down all your negative thoughts, reasons why you wouldn't do it, and then try to come up with a counter argument. Ask us for help with this if you need it. It's difficult, but something you have to do if you don't want to keep missing oppertunities.