You know, having some kind of handicap that forces you to play a certain way is probably the best way to get a style all of your own. So, I would say definitely don't quit, just take it easier and have a think about working around the techniques that cause you problems so you can come up with a result that suits.
Re your actual playing, have you tried fretting the left hand with the absolute minimal pressure required? A lot of people squeeze the neck and end up with their fingers on the actual fretboard. If you look at a lot of really great players you will notice that they barely move their fingers at all, up and down about 2-3mm is about it with very little pressue on the strings. I used to play covers and scales and runs all day long for about the first 5 years or so, but I only found my own niche and actually improved when I quit all of that and started playing what was in my own head (no more pressure to get something sounding like how somone else played it).
One good thing about this is you end up doing your own work arounds for techniques you can't manage. This might help you to reduce the cramp and stress on the forearm and fingers. As another poster commented, playing guitar is great for keeping the left and right sides of the brain firing. You could always call playing guitar physio, great excuse for playing for hours on end;-)
If you have your own way of doing things and you like real heavy music, then just work out what you can hear in your own head. Who knows you could end up with a bizare crossover call Morbid Floyd!