Canal root retreatment or Extraction, considering Meinig - Price theories?
 

I have a molar that has been annoying me for 15 years (I'm 33 now). Everything started with a small and superficial cavity; despite the reduced size the tooth was really painful. After several weeks of medication without improvement the dentist decided to perform a root canal treatment, even if the nerve was not involved. The pain decreased, but never went away completely for all of these years. What I feel is often a pressure more than a pain, and It's getting stronger after physical activity. Last year together with my dentist (a different one) we decided to put a crown. When she opened the tooth, she said that the root canal treatment looked good (only with the eye, no microscope). The crown didn't change the situation, and the same feelings of pain and pressure remained till present in my tooth. In the last weeks, this constant pain-pressure is getting more persistent and stronger. My dentist suggested me to do a root canal retreatment with a specialized endodontist and microscope, to see if there are any roots that were missed.
My fear is that even that procedure won't succeed, and after the money for the crown, I'll waste also the one for the retreatment. Are there high chances that also the retratment could fail? Could it be better to extract the tooth directly?
I say this also because after reading of the studies-theories of Dr Meinig, Price, etc I found out that my general health problems, with symptoms similar to Multiple Sclerosis that I have experienced for the last 2 years (discussion here http://neurotalk.psychcentral.com/thread187613.html now I'm treating it as mercury toxicity), might be due also to the root canal infected with bacteria.
What's your opinion about this?
On this link it is possible to see an Rx taken just a few days ago.
http://postimg.org/image/s9iov8pad/

Thank you
Davide