The only thing the dentist may have done wrong was not given you an antibiotic with the referral to the endodontist. The dentist was correct to send you back to the endodontist to treat the abscess. AS the infection seemed to be controlled by the antibiotics prescribed by the hospital and you don't mention seeing the dentist again, I do not see that the dentist was at fault here. You don't seem to have seen the endodontist for re-treatment that seems to be the reason why the infection returned. Based upon the information you have provided I do not see any grounds you would have to sue any of the dentists involved in your care. You should go have that tooth treated by an endodontist.