I have had a 3 crowns fitted. There is a gap between the gum line and crown. The others hurt when I bite.

The top crown was fitted and kept falling out a post was put it but the crown still fell out. It was cemented back in but now I have a lump on the roof of my mouth that comes and goes. The lower crown that was fitted sometimes hurts when I bite and the third crown that was fitted there is a gap from the gum line to the crown.My top jaw feels like I have jaw ache and I get a horrible taste in my mouth and suffer from headaches. Please help as I don't know where to turn.