Man,can I ever relate to this. I've got short fingers and i'm always muting a lower string. But, as all ready said, practice practice and then practice some more. It's slowly getting better. Also, i've noticed when I have a problem if I take a short break and come back to it all of a sudden I seem to have it memorized. Play things s-l-o-w at first and later pick up your speed gradually with the use of a metranome. Like 5 beats per minute. Trust me, it works.