How to deal with a change negatively affecting my life?

*VERY LONG PLEASE READ* Specifically a person who has changed and become something negative for me to be around but also can’t at this point be away from them?

Everyday stress gets just a little higher, hatred grows a little bit more and then the depression of not being able to change them or the circumstances. It’s become an almost impossible circle.

My father whom I am a carer for and also live with (can not change this at all) I’m expecting my first child, 24 weeks as it stands. My partner lives with the both of us and was my dads idea, we were all extremely happy with each other as although I’m his carer my partner can do heavy duty things like gardening, DIY and drive, etc. It means I can go out and get my dads medication, shopping, basically everything.

I became my dads carer 3 years ago when he got cancer and is now cancer free, in remission. He was pretty much the greatest human on this planet till about a year ago, he fell for a girl who is 28 and he is 54, she’s from a bad neighbourhood, takes cocaine, does prostitution and sells drugs, has her own 2 year old, etc. She isn’t interested in him one bit and because she knows my dad is besotted, she keeps him on a short leash. It works, he does it and knows he’s being used. Doesn’t mind because he stalks her and talks about her every single day. Through this unhealthy attachment he’s stopped eating properly, does weight lifting 3+ hours a day, does nothing in the house, makes mess and sits there expecting me and my partner to do it all. I know he doesn’t need a carer, he’s perfectly able to look after himself but chooses not to as he gets so high on weed that he only recently started doing because of this woman and being depressed he can’t have her. My dad has developed body dysmorphia and uses his no longer cancer to get attention and sympathy. He still tells people he has it when he doesn’t, still uses it to get away with shitty behaviour and being lazy. Whilst being pregnant he smokes in the house after telling him not to, he leaves tobacco and weed all over surfaces and the floor, he eats mine and my partners food, won’t replace anything and we have to literally keep this house going. It’s killing us, what can I do to keep myself going? Please don’t suggest moving out because I can’t, we have tried and we will be homeless.