Hi Kernot
That ‘failure' feeling is awful, isn't it? Rest assured, it is all part of the process - God only knows why!! 
I agree with CKLD, in that if the Femoston was starting to work, don't listen to anyone - if it works for you, that's the only thing that matters.....
You will feel as though you might have gone backwards when you swap from one hrt to another, as it does take a little time for your body to adjust which is why they say to give each one a good three months...
Maybe the progesterone in the Femoston was a better one for you? I do know they give Femoston to women who get horrendous PMS in their child bearing years, and it seems to help with that....
Or you could try the 50 patch? I didn't get much relief from 25 or 37.5, but find 50 is better. My low mood wasn't lifted completely though, so I opted to go back on an anti depressant I'd taken for years (like you I suffered with bad PMS) until about 9 months before this meno hell hit....Lots of ladies here take both, and in my case it was the final missing link to help with those horrid lows. 
Anyway, keep browsing round - so much great info on here. 
Just a thought, too - do you ever get any respite from your child? It's hard enough being a single mum, but if you have a special child it makes it that much harder. Try and take some time just for you if you can - get your hair done, have nails done, even a quiet bubble bath with a book can do wonders for how you feel x