Sounds like you have a significant issue. I'd strongly suggest get an older frame with fiberglass and stringing it with gut. That will be the softest most comfortable setup. The only thing you could improve on from that would be if you have technique issues. You might want to have someone take a look at your strokes in person to see if there are any obviously fixable problems. The three most common things that come to mind for me are hitting late, using the wrong grip, and mishitting.
I don't think a one size grip difference up or down is any cause for concern. If you use the wrong grip for your shots though, that can definitely contribute to issues.
You might want to seriously consider not playing at all for a couple of months and doing some basic rehab exercises. It can be really hard to heal if you are playing and even just subtly re-injuring yourself over and over again.