Yeah, using your middle finger to push the index down will limit you - for instance, you can't use your pinky on the third fret of the E string to play a F major 9th chord.

Try playing the shape higher up the neck - say at the 13th fret. It's still a F chord... the string tension in the middle of the neck is lower so it'll be easier. Then as you build strength, you can shift it down.