cmac, I'm in the dental business and what youv'e experienced is some bad dentists.Crowns or bridges should last at least 10 years.These bridges that you've had did not fit very good therefore they leaked, creating tooth decay.Once those bridge abutments fail the bridge will not stay in place. I would not reccomend going to a teaching school for such extensive work. Go to a good oral surgeon (with refferals).
As Far as root canals go it's kind of hard to mess up a root canal. The nerve is removed from the pulp chamber and thats it. If they dont' get all of the nerve then you may still experience pain. Once a crown is placed over a root canal it should last a long time unless again the crown is leaking creating tooth decay under the crown. After you get your teeth restored, practice good oral hygiene.
Remember...Ignore your teeth and they'll go away.