Of course you can! You can do it! Your body is amazing, and you've birthed before, so you know you can relax and trust your body to do its thing again!
I had two drug free births, and I felt the same way you do, don't know why the doubt crept in second time around, but it did. In the end, it was easy, I had no reason to doubt. You won't either!
(And if you do have drugs, then that's fine too.)