People underestimate the "totality" of the whole personal health package. Poor health in one domain leads to poor health in another.
Major surgery, any sort, can lead to prolonged mental health issues. Just getting over the full effect of a full general anethstesia can take 12 months. Then because you are in recovery mode, physical fitness declines. You're then on a downward spiral!
I've had mates who have had major problems after surgery, depression, bi polar onset, weight gain, the works. 12 months later, when they came out of the initial drug induced torpor, they asked, "I did what"? How did you put up with me? All I said, was you're a mate, so I put up with it.
Unfortunately, many GP's don't work on the total package, they concentrate on what their area of knowledge is.
I'm by no means saying ignore your GP, but there are other wholistic options out there. In my case, my best freind was my chiropractor, who at least got my skeletal frame back into a reasonable condition so I could at least exercise a bit and keep the weight down. Being able to walk and exercise helps me keep other chronic disease factors at bay.
I'm not saying go out and look at mystical practitioners, or witch doctors, but at least have a talk to your GP about what allied health professionals may help.