Lack of energy is the worst side-effect of HT for me. A couple of weeks ago I went on a photographic holiday in Yorkshire, and walks which before my HT started I wouldn't even have been been breathing heavily after, left me an absolutely quivering wreck. It's not nice at all, although I know it's a vital part of my treatment. As others have said, I think you just have to accept that it's "the new you" and go with the flow.
All the very best,