I have a cut on my vagina that won't heal?

I don't have any idea what your lesion was caused by, but i can tell you that it's completely normal that it is taking a long time to heal. That is an area that is always moist. And when something is never dry, it take a lot longer for it to heal. Kind of like a cut in your mouth. I would, however, recommend you start wearing underwear again. Plain, cotton underwear might help protect the area from further irritation. If the problem persists, see another doctor.