@SameKeyThough I'm really sorry you are having such a tough time. :(
One thing that some specialists do, is suggest estrogen-only for a month to see how that feels, and get the dose right. I am not suggesting you do that, but it can be very helpful.
FWIW I have almost no 'brain effects' using Utrogestan vaginally. I've used it that way for some time now after years using it orally.
Orally, I'd feel tired for the morning but it would wear off by lunch. I'd also by the end of 12 days feel a bit sluggish - exercising was that bit harder.
Now though, I have only 1 side effect and that is feeling hot at night when I use Utrogestan. I try to override this by upping the gel a little for a few days.
I suggest you really try not to predict things that may 'go wrong' as there is a very strong 'nocebo effect' (opposite of 'placebo') where you can end up having side effects as you expect them.
Also, although Utrogestan has become the go-to progestogen now, it was rarely used in the UK until a few years ago. It's all a bit 'new'. It does have advantages- for long term use it's supposed to have less risk of breast cancer. But there are other types of HRT that women can take that may suit them better.
For example, Femoston ( tablet form of HRT in a pack) has a progestogen that has the same safety profile for BC as Utrogestan but many women find it has fewer side effects.
I'd not get hung up on using Utrogestan if it doesn't suit you. be prepared to try other types and discuss with your GP after doing your own research.