Reread what Pat B said. You're probably gripping/closing the drawing hand as you draw, pulling the arrow off the shelf. Your bow hand fingers, if they move at all, should move in just the opposite direction. The draw should come straight back, of course, but if anything, your string fingers should relax/unfold a little as you draw/reach anchor.
Folks new to finger shooting often fear that the arrow is going to fall off the shelf so they grip the string/nock ever tighter, which exacerbates rather than relieves the problem.
Regarding the amount of pressure applied by the top finger. For me, it's doing as much work as the bottom two fingers. The reason you don't pull the arrow off the bow when you just let the index finger sit on the string is it's a lot more difficult/uncomfortable to "grip"/close those two lower fingers on the string unless you also close the top one.
All this being said, i agree you should just shoot a bit and not worry about where you hit. Before too long, you'll develop a feel for how to hold the string.