I have cavity in middle of my two front teeth, and they are very wide. What is a solution?

The likelihood is that the filling needs to be shaved down a little and you should start feeling relief. If this does not help then there may be a void in the filling which means that the filling needs to be redone.

Yes, white resin can be placed over teeth. I wouldn’t recommend it on several teeth. Porcelain veneers are a better product, aesthetically pleasing and last much longer than resin bonding.

The answer to this question depends on the current state of your lower teeth.  If you have a significant amount of wear or they are breaking, then I would consider having them crowned.  If not, I would leave them alone for now.  Good luck.