Such kind of lump can be due to a cyst or a benign skin growth. Since it is hard and grows back every time you get it removed, you should get a biopsy skin done from your dermatologist. If it is a benign skin growth then it may need complete surgical excision and removal and the sample sent for histopath examination.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your dermatologist. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.