Ya just gotta keep playing it. I've been playing for over 20 years and "bar" chords still fatigue my left hand when I have to play a lot of them in a song. "F" is particularly tough because it's in the first position where the frets are the farthest apart. It's right next to the nut where more hand strength is needed to press the strings down anyway. This all ads up to a big "Ouch!"

It helps me to think of the strength or "energy" as coming from my bicep as opposed to the wrist or hand. Be careful with the hand cramps. Take frequent breaks to rub your wrist, gently shake it and then resume practicing. NEVER play through a cramp. Tendinitis is not something you want to have to deal with.

As a professional player for many years now I use very few full bar chords. I'm always looking for smaller voicings that "sit" in the mix and compliment the other players and the song.