How long will I grieve for one of the animals I have loved the most?

My cat which was very dear to me was put to sleep this afternoon, every time I think of him my eyes tear up and I feel like I'm about to burst into tears. He was only about 2 years old, and we loved each other so much. I could call his name and he would come, he'd jump up to me and rub me until I pet him. He was always at the end of my bed when I went to sleep, and because he's not I know going to sleep tonight is going to be difficult. My sister would always ask me why Hobbes wouldn't come to her, but I could call him and he would come. I realized today that it was because he was the first cat I have truly loved and he loved me back. What made me feel terrible was the fact that we were at the vet and I would pet him and he wouldn't purr or move unless I scratched him under his chin which was his favorite spot, but only then would he look up and look me in the eyes. I loved Hobbes so much, and I can't understand why he died, and the entire day I have been on the verge of crying, someone was on the phone with me telling me how sorry they were and they would say Hobbes and I started to choke up. I am so sad that i am crying while typing this, and I don't know how to comfort myself. The only way I can think is to love Hobbes' cousin which is almost half a year old and regain that special relationship. I really don't know what to do because every time I think of the word Hobbes, I tear up and start crying. Please how can I help myself from feeling this guilty of not spending his last few days with him, it makes me feel awful that I could have done something and I didn't. I really wish I could go back in time 6 hours so I could still pet Hobbes and he would acknowledge me. I loved Hobbes so much, and I was filling up his grave with tears as I knew I would never see him again. As I type this I look at the end of my bed and see that he's not there, and the tears keep coming. I can look closely and still see his hairs and I only wish he was still sitting there so I could play with him. I love you so much Hobbes, and I hope you knew that because I would sit there hours on end petting you and loving you! I loved you more than several people I know, and you were truly one of my best friends! I love you!