I'm sorry to hear about your bad dental experience!
As the others said though ... at least it is over now and you got through it!!
I had a root canal at the beginning of this year when I had the abscessed tooth and it was really painful! The Dentist had to inject me 7 times before the anaesthetic began to take affect! With my RSD (which is similar to MS), I often need more anaesthetic otherwise it doesn't take effect. My Pain Management Doctor said that I need twice as much anaesthetic as a normal person.
The Dentist kept hitting my nerves and it was really painful! She said though that a normal person would just scream and go running out where as because I have chronic pain, I don't feel it as much and can sort of 'tolerate' it as it's not as bad as the pain I deal with on a daily basis!
Once the root canal was over though and everything settled down, everything was OK and I felt a lot better for it!
I hope you feel better soon!