I agree with BrowersBlues tell someone in real life. BrowersBlues said If I was a friend of yours I would be outraged and would want to comfort you. Could you tell someone just to get some support for yourself? I know your dad is critically ill but that doesn't mean that you don't need support too.
Yes, Yes and Yes again. You need support too. Your dear dad would want that for you. I lost my dear father 10 years ago and my mum was devastated but not once would I have treated anyone the way this man has treated you.
Whatever happens I really feel you must tell friends in real life. If you really feel your father will be upset, if he has no idea what your dp/ex is like, then just swear any family or friends to secrecy around you dad.
You need support because people will presumably be thinking your partner/ex will be supporting you at this time and if he is not - which he is clearly is not!
then you need support from others.
You do not mention any other family? Is there anyone else, aunts, cousins etc? Are there close friends you can turn to? Work colleagues?
You also say DP is a kind and well respected man and his family are his top priority.
I cannot see how he is kind.
And most scarily …he doesn't see it as a whim, he sees it as me being selfish and immature about his needs and therefore I suffer the consequences of being unsupportive towards him.
So how does he see this all panning out, you come back humbly apologising and he lets you stay or you just split up?
Run for the hills seaside. You are well out of it. Build a life for yourself maybe you will meet a partner who is mature enough to care for you.