An abcess is indicated by clinical symptoms, swelling,purulence,temperature etc. If the
root is infected then I would think you need
root canal therapy--a tooth can be sensitive to biting if it is near the sinus and the sinus is inflamed. These are two totally different conditions with different treatments. I can not tell which one without examining you.Ask your dentist if the nerve of your tooth is dead, in that case you would need root canal treatment.