Try looking at Learning guitar with a wonky digit or 2.

There's also a thread (see the list of related threads up at the top) on instant calluses. I don't know if that would help, because I don't really understand the problem.

If you're missing a callus.

If you're missing the fat pad between the skin and the bone, there's no amount of callus that will help, because then the callus will press against the bone and hurt.

If you have a mushy finger (*), I'd suggest open tuning or adapting your technique. Taping your finger so it doesn't squish might help, or some sort of leather and elastic prosthesis. It will interfere with your sense of touch, but if the tip's alread numb and you've learned how to find the strings without feeling them, it shouldn't be too much of a change.

My problem's likely different than yours, but I play a 3-finger C using index, ring and little fingers. I can do a full or partial barre for the F.


(*) All obvious, overused jokes about digital erectile dysfunction and Viagra hereby pre-thought of..