Unfortunately, it’s impossible not to make mistakes. In fact, the sooner you stop worrying about them the quicker you’ll limit them. It’s inevitable in your banjo playing growth that you’ll have to make some noise before you can make music. This may hold true even more in speed building. Go for it, make some noise and clean it up when you gain more control. Practice is practice! It’s not a performance! No rules! Get after it! When you achieve some proficiency, then you can perform and do your best to play music that is pleasing to others and to yourself.