Weight gain help, please

I've had a problem with my weight for the last 3 years and am currently under a dietician, but no matter how hard I try or how much I eat putting on weight is always a problem. I have a protein shake in the mornings to give me extra calories, etc.
I've had knee surgery which caused me to lose more weight due to being in so much pain afterwards and I currently work a couple of hours at a stables few mornings a week.. I have attempted to go back to the gym even just to work my upper body, but still due to the pain this hasn't been very successful (I am on very strong painkillers or nerve block tablets)
I can't seem to let go of that fact that's it is okay to have a 'treat' once in a while and I won't really do much harm.

Can anyone suggest or give me any tips on how to much some weight on without getting 'porky'? Thanks