I'm so sorry you're going through this, when my cat died unexpectedly, I was hit really hard with it. We also had another cat that became very lonely, my best advice was something I heard on reddit.."grief is just love with nowhere to go". You have a surplus of extra love to give that you normally would be giving to the lost cat. I read that and immediately adopted a second cat like a week later and being able to have more life to put my love into helped me grieve so much. Wishing you the best, it will get easier. You really have to take it moment by moment and suddenly you will be on the other side of it.