I've found the difficulty with weight loss is psychological more than anything. I know how to do it and I've done it before, but I'm quite overweight now and until recently I've been unable to do anything about it. Losing weight while depressed has been beyond me, even though its slowly been killing me (and I'm not exaggerating).
I've no doubt its easier to lose weight before starting HRT, but its not necessarily difficult afterwards either. I'm losing weight it rapidly right now, and the only difference is I'm no longer depressed and I have some goals I desperately want to achieve. I'm actually on a much higher does of HRT than before as well.
Obviously this is all very personal, and I've no idea how it relates to you. You might find starting HRT makes you feel great and want to continue with the weight loss or if the idea of starting HRT is motivating you perhaps it would do the opposite.