I think most of us split-finger guys most often have the other problem, building a calous on our third finger.
I attempt to place a little more presure on the index finger, but not over doing it. In doing so, I get a more balanced draw.
If you index finger is sore, and is sore on the part where it touches your arrow nock, then you might have a high nocking point. Maybe your shooting style causes it.
Are you shooting a tab? If so, has it got a spacer, or leather to protect your finger? Maybe you need to go to a thicker glover or tab.
I will be waiting to see what the other guys/gals have to say. I will be watching this with a great deal of interest.