Being afraid of what others say or think about us can happen pretty often at any age. We want to fit in but also be special and stand out.
You decide if you want to do it or not! I think it's important that first of all YOU feel comfortable about paining your nails and the other "girly things".
There will always be people that judge us and others who accept us but that's not up to us, it depends on their personal standards and "rules". We can't change that. But you can make sure that you do what's good for you, for your life and future and you're happy with yourself and what you do.
There are many ways to calm your nerves like deep breathing, looking at a beautiful picture or seeing something nice around you, squeezing a stress ball, listening to a song or any sound, a nice taste or scent...or whatever works for you and calms you. The point is to get distracted from your anxious thoughts and be more present.
Smile :) ! No matter if you paint your nails or not you are an amazing person and everything is ok!