Maturity

When you stammer what age do you feel?

Try reminding yourself that you are older, wiser, more experienced and that you are pretty good at handling yourself in many different contexts and use the generalisation as a setup for every experience in the future.
If you are unsure how someone of your age behaves there are no end of role models in literature and TV. Remember you are not being someone else you are still you but identifying with other qualities and behaviours.