What it sounds like is more of an issue with the crown than with the root canal. Maybe I over looked it, but I don't remember you saying how longer ago you had these procedures done. Being that you aren't having any hot or cold sensitivity, I don't think it sounds like a reinfection of the tooth, or a failed root canal. Pressure sensitivity normally indicated a crown, which you already have. The pain you are experiencing isn't from the root or the nerve, but instead the tendons that hold the tooth in place. When they are irritated, you get this type of pain. I don't know if you know anyone with braces, but I am sure they will tell you the exact same thing. When braces are first put on, or when the wires are adjusted, sometimes even a tongue slightly touching the tooth will cause pain.
My impression of your new dentist saying that you need to have the mesial board checked on the crown indicates an open margin. This means that the crown is not fitting properly & may let in bacteria which cause decay & then cause more problems.
Like I said before, I don't see where you said how long ago you had this procedure down, but it may be just now that things are returning to normal in your mouth, or that your teeth are shifting & that it why pain is starting. It sounds like, if it is possible, you need to see your previous dentist & have him take a look (as far as replacing the crown goes). If that is not possible, I would look into your options & more than likely plan to have the current crown replaced to ensure that recurrent decay does not become an issue.
Have your dentists ruled out infection? I've been having trouble with my # 30, having had a root canal 2 1/2 yrs ago. My situation is different than yours, but I did end up with infection after dentist inserted a titanium post to hold my crown on (seems old bacteria may have been stirred up). Now I'm holding my breath and hope all is well after dealing with two different antibiotics. I can't offer alot of advice, only that I understand what tooth pain is. Perhaps you should consider an opinion from a well experienced endodondist. The dentist saying you would "have to live with it" just doesn't sound right to me. Good Luck!