I have had the same problem for years, ever since I started jumping, so I've kinda figured out what works for me. Aim your horse for the jump and then just clear your mind. Don't think about anything.. don't think about how many strides left to the jump, don't think about your position, don't think about when he's going to take off or what it's going to feel like. Just sit there. You might find yourself automatically going into jumping position over the jump, you might find yourself getting popped out of the saddle, but either way it doesn't matter... YOU DID IT!!!! Now that you know you can do it, take the jump again, this time focusing on a nice smooth approach and giving a nice clean release over the jump. Each time you face a new jump (new height, scary filler, etc) take it the same way, first just seeing if you can jump it.
Every ride, good or bad, teaches you something new.