OP, like others I do understand where you are coming from. I spent 10 years in a relationship I knew was a dead end from year one. The problem was we were at different life stages, XP wanted to settle down and do the pipe and slippers thing and a wife with a nice part time job and maybe children. I wanted to have fun, be frivolous, had ambition and a career that meant working very long hours. I wasn't available enough so he punished me by sloping off for months on end and I let him come back, that was his punishment, our collective punishment was each other. So it became a competition, the more dramatic his leaving got, the more distant I became and it never made either of us happy and only drove us further apart. If I have a regret in life it is my part in that colossal waste of time and pain for both of us.
It seems to me that you and your DP are at different life stages too, you never will reach those stages together. She has a nice comfortable life living at home with mum, you don't enjoy it and it's driving you mad She has a lovely DP who only needs a bit of attention and he helps pay all her debts off. She has no reason to leave her comfortable life and has even threatened you with a split if you want to live elsewhere for a while. That's not love, it's control and you are on the hook If she loves you and vice versa, there won't be a split or it will only be temporary. If you split for good you've lost absolutely nothing but a drain on your resources and you will gain your peace of mind..
Don't let it run on for years until one of you literally stops caring or has a breakdown. One of you needs to make your mind up and it won't be your DP, she has everything in her life exactly where she wants it and is fully comfortable with the situation. This doesn't make her a bad person we all like to be comfortable but you want something different to your DP. If she cannot see how this is hurting you and won't work in a compromise you are both happy with then she never will and she doesn't deserve you. You both deserve better but you have to stop playing the game to get it. Partnership is never a competition someone will win, it's listening, reflecting, communicating and coming to a compromise you are both happy with. This is no compromise she is winning a game and you are allowing it.
All you have to do is move out to your flatshare, you don't owe anyone any part of you that you are not willing to give even if it is a reason.,You want to be happy and this is not what happiness looks like for you. If you can't have the discussion because it is too difficult for you, then just leave. If she wants to know why then just ten words will suffice ..."I don't want this anymore, it doesn't make me happy" end of conversation. It will come to that in the end anyway, leaving a situation always does boil down to those ten words.