I have had several decaying teeth and I find that what has worked best until I had them removed was using DenTemp o.s.
It's a crown or cap emergency filler. It's a paste that hardens and has clove oil mixed in with the paste. It comes in a package on the tooth care section of most pharmaceutical stores. The package has a little blue container filled will white paste.
Take any OTC meds beforehand. It's recommend not to drink or eat for about an hour after.
This works great if you have a large whole or broken section of tooth. Just brush the tooth well. Use mouthwash. Just make sure its clean as possible. Wet your figer tips a little. I just lick the pointer and thumb tips. Get a small amount on your fingers. Roll into a ball and try and pack the missing section as much as possible. I've experienced pain sometimes depending how bad the tooth is damaged. But the clove oil will numb it soon. Once inserted bite down like normal to help mode any extra paste. Don't drink for about an hour to make sure it hardens.
It will last for about a week and will help reduce pain big time.