Make shure that you don't try to get all the power from pressing your thumb and first finger together... this is hard and your hand is getting fatigued very soon. So... here's the secret: pull your whole arm back (from your shoulder). Make shure that your posture is good and your right forarm is in front of your guitar body to keep the guitar in place. Done right, you could play the F bar chord even without your thumb touching your guitar neck. (of course you don't play like this in real life, but practice it to get a feel for "pulling the arm back")