Crown leads to a Root Canal

Hello,

A few years back I had a Crown placed on my right bottom tooth.  I don't think the fitting was right, especially when the Crown was placed on, I was never told to bite hard so the Crown was a bit higher after the cement harden.  The dentist then smooth or lowered the surface with some kind of sanding equipment (sorry I'm not sure what was used).  

I detected a bad smell when I was flossing about 6 months ago and I thought it's just because I'm not flossing good enough.  I tried to floss harder and the Crown came up.  It hurts really bad and my tooth was extremely sensitive.  I pushed it back down and it seems to be fine.  The bad smell from flossing have been increasing over the months and I finally went back to the dentist office.  

The Crown was lifted and it smells really bad.  I had to get numbed because it would hurt so bad and no one could even touch the tooth.  I was seeing by another dentist from this office  and he advised me that a Root Canal would have to be done due to how sensitive I am now with this tooth and also a new Crown will be made.  I will have to pay for the Crown because it has been less than 5 years.  My total bill is about $1000.  

I am just really confused and not sure of what to do.  I don't know much about root canal or crown so I have been reading on the website to see if I could find the answer.  I did help me to understand a little bit more, but I'm still not cleared on my situation.

I would really appreciate any advice from a professional that will help me with this.  My appointment for the root canal is this Wednesday.

Thank you!