Actually do Ameaturs Mind as a prerequisite but seeing you're 300 rapid, are you sure you need to work on positional?
I only play Daily games, including Club matches. I'm 80 years old and my brain does not process fast enough to play rapid time controls. I also have low vision, so it takes me a few minutes just to assess where the pieces are on the board. I'm currently 945 on Daily and my goal is to reach 1200 before I die. Hopefully, I have enough time to study a bit of positional chess ...
Hey Snowchlobe, yes, I moved here a couple of years ago! Mostly I'm at uni in Southampton though. There is a decent amount of chess here and some strong players, including an FM! There's also a new Isle of Wight International tournament next month I entered which should be good fun. Do you play in person a lot?
Endgame_Horizon, with regard to those two questions, I can only say based on my experience that I improved gradually... I 'broke free' from the trap by (a) playing rapid games and using Game Review, (b) Chessable courses, particularly on positional thinking, and (c) playing a lot of tournaments OTB and fixing all the gaps in my knowledge. Think of chess improvement as diminishing returns - you will never hit a 'maximum' rating, but improvement will just get asymptotically slower and slower.
I think the psychology factor is what's relevant here though. It sounds from your message like you are very frustrated, referring to the rating trap as a 'hell' Just remember chess is a game, and there can be too much of a good thing. I've been in that position before frustrated I'm not making progress. Try and balance it with other hobbies; I limit myself to 3-4 rapid games a day, because I'm fresh and can properly concentrate, whereas more than that I get tired and I can't calculate properly. Plus, if you get bored of the actual chess and are just watching your rating at the bottom of the screen, of course you will pay less attention to the game and will blunder more.
Furthermore, the 10-0 time control is extremely unforgiving. It's more like Blitz than rapid, especially in endgames, so if you are set on playing that proper time management is crucial.