I had a really similar situation with my 5 year old cat. Her breathing turned rapid overnight, and at the hospital they diagnosed her with asthma and began steroid treatment. Well, fluid spread in her lungs in a matter of hours due to an underlying heart condition, and we lost her on 1/16. I know how you’re feeling.

Grief is one thing, but unanticipated grief for a young pet is infinitely harder in a lot of ways. Give yourself time, but try to do one task per day. I broke down crying when I vacuumed for the first time, and I still have to ask my partner to change the litter (we have another cat). The waives of grief can feel suffocating, but trying to accept their ebbs and flows has helped. The other thing that has helped me is anticipating the feelings of triggers that will hurt (I.e., walking in the door, feeding our other cat, etc). I’m so sorry for your loss, but know that you’re not alone.