Problems with crown portion of implant

(Toronto, Ontario, Canada)

I have Problems with crown portion of implant:
The second molar at the back and on the bottom of my mouth was replaced with an implant. The crown was attached last year and fell out about 1 year later.

I went to my dentist immediately and she reattached it but it fell out again after 6 weeks. I had it reattached again but it fell after 2 1/2 weeks, a couple of days ago. The crown has a metal column that goes over the implanted post. The column has a sharp ridge that digs into my gum and causes pain. I have not done anything about the crown this time because I don't know what to do or who to go to. Is there any reason I should get it reattached immediately?

No one told me that I would be in pain for the rest of my life with an implant. The old tooth had had a few root canals and the crown had cracked and fallen out. My dentist said that there wasn't much tooth left and if she replaced the crown, it would probably last only a year. It's at the back of my mouth and I haven't been able to chew much with it for decades so I was willing to leave the space empty but my dentist said that the other teeth would shift around over time and cause other problems. Is this true?

Is the crown defective? Should I insist that they replace it? I don't have dental insurance and paid for it myself but I feel as if I am being bilked. Should I consider it to be water under the bridge and go find a new dentist? My dentist says that she used a temporary adhesive but threatened to use a permanent one if the crown came out again but I don't want that if I am going to be in pain again.