Is this on your lower back, near the buttocks in the middle? If so sounds like pilonidal cyst which often infects and is indeed difficcult to get rid of -- and once gone, can later recur.
Treatment is antibiotics, surgical drainage and in some cases surgery.
Whrlen you see your doctor ask:
(1) Is It a pilonidal cyst?
(2) Given how long it has persisted now, is surgery indicated?