I very much know how you feel..though i wish no one did.
One thing that might help (later) is something someone else told me when one of my friends was about to go.
Basically, i spend a lot of time in my head in general, and even more when upset. As such, the time i was spending with them, i kept thinking about losing them and how much i was going to miss them and the times we'd spent in the past. But what the person told me was to really try to spend every minute i could, there, in the present, with him. How he looked, how soft he was, how he smelled, the little sounds and faces he made..absorbing every inch and every moment .. Being present in the present, not in my head. And, i did that, and when i had to let him go because his pain was great and though i wanted him to stay forever, it was best for him, i again started to drift into my head and how wrenchingly crushed i was...and remembered..and breathed.. And tried to let everything go but what was there that moment...with him. And how very deeply i loved him.
After he was gone, i had loads of time to cry and miss him and think and be stuck in the past and future..and i did. But i'll always be really grateful the guy told me that because it gave me moments that will be with me forever that i otherwise would've missed.
I'm really sorry about your kitty 
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"When does the human cost become too high for the building of a better machine?"