Thanks for putting up your question on Just Answer.
So there are two main issues :
1. The tooth coming in under the root canaled site:
This could be a supernumerary tooth. This is quite common and more often than not, such teeth are impacted. That is, they do no erupt in line with the other teeth. They need to be surgically removed.
2. The bridge failing :
There are numerous factors on which the success of a dental bridge depends :
- the teeth preparation technique
- the quality of the work of the lab
- the cementation procedure
Without knowing how an of these steps were done, it is impossible to pinpoint any one cause.
I'm sorry that you have had to endure so much dental trouble over the years. All I can say is that the supernumerary tooth is an unfortunate coincidence, and the failed bridge can be a error.
Hope this helps. Please leave a positive rating if my answer has helped you.
I do not get credit for it otherwise!