@lindaray Welcome to Mayo Clinic Connect. This all just sounds so uncomfortable! I am tagging @loribmt in this response. She had a career in the dental field and may be able to help you understand what is going on, plus give you some pointers.
Ginger

Hi @lindaray. You’ve certainly been through the wringer with little ‘ol tooth #3 with a new crown, root canal, an apioectomy and the possible side-related issues in your mouth. You gave very detailed and helpful accounting of what’s transpired over the past year with that tooth.

With the deterioration of the tooth under that old crown from 20 years ago and the pain you were having in the tooth, it’s a possibility that it was already in the beginning stages of dying. It can be difficult to diagnose at that stage and often doesn’t show up on an xray at the apex of the tooth until there is an actual infection. So your dentist, at the time, may not have been aware.

When the crown was placed and it felt off, it may have been due to the tooth (nerves in the tooth) being dead. The pain associated with it could have been from the abscess forming on the apex of the tooth and the tooth might have felt ‘off’ because of lack of sensation with the dead nerve tissue.
It sounds like the infection with the root canal was a little challenging with its recurrence in one of the roots, requiring the apioectomy. Were you given any antibiotics at the time of the root canal treatment or when the abscess was first discovered?

Fast forward to now, with the symptoms of dry and burning mouth that you’re dealing with. That’s a miserable condtion!
I’m not a dentist but as @gingerw mentioned, my career was in the dental field for 30+ years. So I’m digging into experiences I recollect to relay some possibilities. While not common, it is possible for patients to have allergic responses to the material used in the permanent crown. Also not common but possible is long term inflammation from the abscess and root canal treatment which is initiating an autoimmune response. I can recall 2 occasions when patients had long term issues where the resolution was the removal of the offending tooth. Which I’m sure you’re reluctant to do because of the money invested in it already. Again, I’m just tossing these scenarios to you as examples of actual patient experiences.

First it’s important to find the underlying cause of the inflammation. My suggestion is to try your primary care physican, a rheumatologist or a Functional Medicine practitioner to check to see if you have inflammation in your body. This is done through simple blood work and then work from there. The doctor may be able to help you discern if there is any correlation between the tooth and your inflammation or if this triggered an auto immune disease such as S’jogrens symdrome. Which a rhuematologist will be able to diagnosie.

If you’re one to do research this article discusses the potential for autoimmune responses with an abscessed tooth. From PubMed:
Root Canal Infection and Its Impact on the Oral Cavity Microenvironment in the Context of Immune System Disorders in Selected Diseases: A Narrative Review https://pmc.ncbi.nlm.nih.gov/articles/PMC10298853/

I have burning mouth syndrome related to a medical condition and when I have flares I avoid toothpaste with mint and use children’s toothpaste. I also switched to a super soft child’s brush. I avoid ice, condiments with acid, salt, or hot spices. One thing that seems to offer a level of comfort is ‘Pulling” with coconut oil. 1 teaspoon or more of organic coconut oil. I let it melt over my tongue then swish vigorously around my mouth for 20 minutes. Then spit into a tissue. Not the drain because it will clog eventually.
I hope you find resolution soon and please let me updated, ok?

@lindaray I also want to welcome you. Lori gave you some great advice. I've got some similar experience myself in a situation with my front teeth. I broke my teeth when I was 9 and had lots of dental work done and redone over the years. Eventually, all 4 of my upper front teeth had root canals and crowns, later replaced by a bridge. I was a kid when the first root canals were done, and 50 years later, they started to fail. Because the bridge tied everything together, when the first tooth sent an infection into the jaw bone and ate away the bone around the root, the porcelain tooth didn't get loose. I'd been feeling pain that I thought was sinus pain, so I missed the symptoms until I felt a soft spot in the hard palette area in my mouth because the bone was eroded. I had an apicoectomy to save that tooth.

A couple years later, a second of those front 4 teeth seemed to be going bad. I didn't want to keep going through this waiting for the next one to fail, so I made the decision to get ceramic (Zirconium) implants and new Zirconium teeth. That has been a good decision, and I am careful with oral hygiene because I know implants can fail if an infection gets around them. It's an expensive process too. When you add up the fees for the dentist, the oral surgeon and the dental lab, it's about $5000 per tooth to be replaced with an implant and new crown.

One question I would ask is if you have had any spine injuries or neck injuries such as a whiplash? I am also a spine patient, and have had pain in my neck and jaw that triggers pain in my mouth that is not caused by a dental problem. It can make you think there is a problem with a tooth when it is the nerves that service the mouth that are getting compressed somewhere between the neck and jaw.