mollymoo, welcome to MM!
If you are feeling flat then it is probably safe to say that your current HRT regime is not working. Having looked it up, it would appear to be quite a low dose of oestrogen and also being an oral preparation, probably not as effective as the transdermal types of HRT like the patches and gel.
I don't know anything about Tridestra, has it been recommended to you? It does contain a higher dose of oestrogen which you need if you are having breakthrough flushes and sweats so you could give it a try but personally, I would recommend the oestrogen gel or patches because from my experience they are more effective than the oral preparations. It does mean you need to take a separate form of progesterone for several days each month, usually Utrogestan, which will provoke a bleed but many MM members find that this combination is more effective because it is bio identical and very similar to the hormones you produce yourself and it has fewer side effects.
It might be worth discussing this with your GP.