i'd also say to practice the same chord shape just further down the neck. You should find it a bit easier. Then continue to practice and work your way back to the F. Also if you get too frustrated you can cheat a bit by playing more of a power chord to get through a song. Just make sure you continue to practice it. It may take a long time (possibly a year or more) depending on how much you practice. Part of it is building calluses in the right spots on that index finger. One of those finger exercisers might also help. i think it's called the grip master. you know the thing you squeeze for finger strength.