bridge failure

My dentist had installed two permanent bridges in my mouth and both have failed.  The first one was about 1/8th inch down from my gum line and he had to fill in the gap with some sort of porcelain bonding material.  After 4 years, the whole bridge was so loose I could wiggle it back and forth and he ended up pulling the bridge and teeth and I now have a partial plate there.   Now the bottom bridge has a tooth that keeps getting infected and he wants to remove it (3-teeth) and replace it with a 4 tooth bridge, pulling the one tooth that's always infected.  It's getting expensive and I am losing faith in his abilities.  Aren't bridges supposed to last and can't they  be removed and just have a tooth added rather than making a whole new bridge each time?