I had a molar ( 3 root ) that was seriously bad, has 1 root canal, cracked down to the end of 1 root. The dental school insisted that it be removed and the head of the dental school got quite upset when I refused. The tooth eroded down to the gum line, then below the gum line but I could tell that it was still there.
After a while, it started coming back, first as two sharp spike and then basically a bit over a half of a molar. The root that had the canal did not regrow.
On the other hand, if you are thinking negative thoughts, then you are likely to have less success in regrowing something.