We're having a similar problem with one of our elderly in-house rescues, so I can certainly understand the frustration.
If you're willing to spend the time, there are homemade recipes offered here, by a vet who specializes in feline nutrition and isn't selling anything www.catinfo.org
Otherwise, keep it simple and don't change foods too often - which can present the problem all by itself even in healthy cats.
Find a canned food that lists the actual meat in the first two ingredients rather than some vegetable product (called something 'meal') or meat 'by-product' which can be anything from hooves to chicken lips Images available only to Customers.
Many have forgotten this truth, but you must not forget it.
You remain responsible, forever, for what you have tamed.