Weight problem

I list 6st with Slimming world then diagnosed with breast cancer March 2017. I had lumpectomy and lymph nodes removed then radiotherapy. I am struggling to loose the 2 st I put on. I’m only 4ft 11ins and weigh 16st. I’m desperate to loose weight but I’m taking Letrozole and finding it so hard . Any help or tips woulf be appreciated. X