If you are in a band, or in a theater company, you do lots of rehearsing. In fact, it's your primary thing. And it helps if you love to do it. I happen to love to rehearse; even when it's hard, or intense, or all-consuming. In fact, it's the all-consuming-ness that's a real kick.
I love to immerse myself in the work. You lose yourself, and I guess in a way you find yourself too. It helps if you love the work too. That way you can enter into a rehearsal with no qualms. Now sometimes this is an act of faith, even if you wrote the song, or the monologue or scene.
Everyone has doubts, all your favorite artists at one time or another have had doubts in the work they are doing. Read any biography about a musician or actor, writer and you will find that doubt is part of their psychic make-up. At least if they are honest with themselves.
That's the key. To fully commit, to steam-roller any doubts, no matter what. To act as if, the work is essential. And by that leap of faith, you make the work essential to yourself, and hopefully to others too.