Hi Yolanda, sorry you are having problems with your couch sagging. Hope this helps you. How to Fix a Sagging Couch: 14 Steps (with Pictures) - wikiHow
Part 3. Fixing the Frame. Check the slats. If the wooden boards that support the couch frame have broken, you will need to replace the wood and screws. Fix bent springs. Remove any broken wood. Attach the new wood to the couch. Put the upholstery and fabric back on the couch.
Add more fill and maybe foam. Find a seam or if lucky a zipper and add to it. Make it uncomfortable for kitty to go back by using a spray bottle to train her to stay off.
If you're good with a needle and thread - and there isn't any other way to get into the cushion, here's what I do. If the cushion is loose, find a seam or zipper and open it up. If not, then find an inconspicuous place and make a hole about six inches long with a seam ripper. If there is already a pillow of fill inside, check out the kind of fill it is and simply add more, close your opening with a blind stitch and replace the cushion. If it is attached to the sofa - a little more complicated but use the same technique. I try to make as small an opening as possible (less than 6 inches) on the bottom or side of an attached cushion because the new seam will take the least pressure that way. (we have big dogs that love to look out the window). Hope that helps.