I have a decently good music voice but only when I'm alone
Are you only good at singing alone?
I'm better at singing when I'm alone
It's fine to sing alone. If that's what you enjoy, more power to you. Singing in front of others is nerve wracking.
I'm too scared to take that next step to sing in front of a crowd.
That's fine. Having stage fright is totally natural. But the only way to overcome it is by practicing singing in front of someone.