Sounds like there is a problem with the fit of the new bridge?
I would seek a 2nd opinion form a prosthodontist ( specialist in bridges) and see what they can tell you about your dental issues and how to rectify them.
My initial thought is that your dentist may be using the wrong type of cement for your crown, repeatedly. Another possibility is that your bite is off. This means that the bridge may not be properly aligning with your other teeth, which can cause the bride to come off. In my 10+ years of practicing cosmetic dentistry, I have never seen this happen to a patient. However, anytime that I have heard of this happening from a fellow dentist, it was the cement that was used. Either the cement is wrong or it is defective. Good luck and I hope this helps. Follow me if you have more questions.
Sarah Thompson, DMD
A new properly designed and fitted bridge should not be coming uncemented twice in a year unless it had been temporarily cemented in order to see how you liked it or how the teeth supporting it are doing. The bottom line is the dentist came up with this treatmant plan and it does not seem to be working. Sometimes the dentist just has to admit it is not going wwell and has to do whatever it takes to make the patient happy. I have had to redo a bridge, but it was for esthetic reasons which is always a judgement call, and not for the design or fit. When treatment goes as wrong as you are describing, most dentist would go to whatever lengths necessary to avoid things going outside of your office and getting real ugly.
Given the circumstances you have provided, I would recommend a second opinion in order to determine if the bridge can be made to work or if there is an obvious need for implant treatment. If implant treatment is needed then you can have the discussion about cost of treatment and if you want to proceed with the recommended implant treatment.