The problem of taking HRT if you have a family history of cancer is very tricky. There is a big scare about HRT as you know and many women quit. I myself have not quit and several of my friends did. One friend who, like you, has fibroid tumors in her breasts for years and years has been taking HRT in spite of the warnings as she could not handle the hot flashes and other problems. So what I am trying to say is, its your decision if to risk taking HRT or not. The risks you know (cancer) the good side effects, no hot flashes and mood swings.
It won't stop you from faining weight. My doctor once told me that you only gain weight from food. Now maybe you do not eat the right foods? Or you do not skip the "bad" foods? Could that be the problem? Usually if you skip bread, pasta, potatoes = starches, and sugars in all sorts and forms, you lose weight.
Have you tried www.flylady.com? If not, go to their web and see the diets they suggest.