WELCOME!
Your weightloss struggles may be because your body is also trying to heal itself after all the surgery/chemo. Radiation can poop you out as well. And depending on what/where, especially if you lost an organ in all of this - your body now needs to accommodate for what's missing and regain it's strength as it heals.
I am a 20+ year survivor. Give your body time to heal and rest. With the diet, maybe approach it instead as adding something that is healthy and swapping it with a food that is less. If you normally did chips, put those away and chop up some fresh veggies that snap when you bite into them. For myself, as I healed, there were so many struggles to feel good about myself when I knew my body turned on itself. I think take it slow - join in the camraderie here (it helps!), know it may take your body time, slowly make changes as dealing with cancer is a big change in body/mind/spirit. The weightloss can follow. Don't forget too if you are on any of the prednisones, that can contribute to wanting to eat ALL THE TIME (at least for me).
Good luck - stay well, stomp that nasty C to the curb and smile big, you have survived the big hurdle, five years will come soon enough to say CURED. My best to you.