That was the same with me after Flo. She spent almost her whole time in my room for the last couple years (she seemed happy til the end btw, I don't mean she was confined to my room because of pain or anything. she was just old). And because of my social anxiety etc I spent more time than most people with her. Slept with her every night etc. It's hard. But it won't stay this hard. I still think about Flo a lot, and evven slip up sometimes and say Flo when I'm talking to Jess. But it's not painful like it used to be.
And MLA is right. It's much better for a cat to live a good long life with people who love her, and then be put "to sleep" in their arms. She will have felt safe with you there and felt no shock or pain that comes with a lot of other deaths
Flo was nearly 14 when she died. Not as old as Destiny but still quite old for a cat (especially as all our previous cats had died quite young after being hit by cars etc, apart from Flo's sister
But becuase I was quite young when we got her (eight) it felt like even longer. It's like it's all you've known, isn't it. So it's hard to adapt. But it will happen. You won't forget her obviously, I'm not saying that. But you'll adapt, and you'll no longer feel the same way you feel right now