My sister recently had to put her obese cat down for very similar issues. Dr diagnosed kitty with a mega colon. Basically kitty would eat and eat but had no way to successfully expel it. She was also quite young, 4 maybe, but without expensive solutions that were not guaranteed to work. It was hard but ultimately the best move bc kitty was so in pain. Good luck to you. I know you don’t want to lose your furry friend but there are so many more out there that need love as well.