I have had a tooth crumble as well - in fact there felt like there was hardly anything left there! Like you, thought the worst, but taking a tooth out is their last option.
They filled my tooth, and it was painless (good old cocaine injection), but even if you have to have more than a filling, ie root canal work, they will NOT take it out unless it is beyond help, and that's very rare...and they will give you as much pain relief as you ask for (I always ask them to make sure they numb it well).
If you're scared of the dentist (as many, many people are) tell the dentist and they will do everything to help, but please go if you need to. They will even numb your mouth for hygiene work (cleaning and scaling) if you ask them to.
Good luck.