I am thinking of not playing music and singing at my great grandmother's funeral
That's too bad, why don't you want to play music and sing at her funeral?
I just don't feel comfortable doing it.
Is there a reason that you're uncomfortable about doing it? I think the family would appreciate you doing this and it would bring them some comfort during a difficult time.
Fuck that, I'm not a good singer and I want to do it my way.
I think you will regret not doing it. You're probably a good singer and it will make everyone happy. Sometimes we need to show kindness to others and show them how much we care about them during a difficult time.