I Have A Small Blister Between My Smallest Toe And

I have a small blister between my smallest toe and the second one I have had it for years now and everytime starts to pain I would leave my foot open for about 3 days and it would heal but now it has a severe pain wich is not healing even after leaving my foot open for days and applying some powder which I was given by the phamacyst, advise how to treat this problem