Doubt's allowed.
It can be a sign this isn't for you, but it can also be a sign of the natural fear of change.
Any change, even wanted change, can be terrifying.
Even people with massive problems, even more massive than ours, can fear what will make their problems better.
Cause that means things change, and the known becomes unknown, and that's scary.
I'd take the time you have to think about what it is that's making you nervous exactly. Maybe things'll make sense better then.