What kind of bit are you using? My horse gets a cut mouth if I use a metal bit so now I use a happy mouth or a rubber pelham and it is fine. Also make sure the bit is big enough and not pinching in the corners.
As for getting it to heal now I think the only thing that will work is not putting a bit in mouth for a few days.
This used to happen quite a lot with my horse and the only way to heal it was to cover it in vaseline, the cut, and ride in a hackamore til the cut is well and truely healed. Have ridden him in a rubber snaffle ever since, and always check that the bit is wide enough. Hated those rubber cheek things and my horse used to just try to eat them!!
What cream are you using??
You can get latex bandage which can help. Be careful of nathe/happy mouth bits and Vaseline as it can burn/rub.
I swear by preparation H gel (piles cream) with my pink skinned mare.