Hey there-

It's possible it's not all "your fault". You might want to have the action on your guitar checked to see if it's set too high. If the strings are too high off the fretboard, it can make it a lot harder to press them down and lead to the hand-fatigue problem you are having. Also, a lighter gauge set of strings might make it a little easier on you while you're building up your hand strength.
Good luck!
Lisa McCormick - Guitar Tricks Instructor