Losing a pet is an incredibly hard thing to go through; in all honesty it takes years to get over... if you ever do.
You always feel a little pain when talking about a cat/dog that died because they played such a huge part of your life... especially after so many years.
I lost my cat Jet a number of years ago, I was in very much the same situation as you as well. Had the cat nearly all of my life, she would wait for me when I got home from school, then college, then work. Sleep on my bed at night, sleep on my lap while playing video games. Its an awful thing to go through.
Time helps, and eventually the best thing you can do is get another cat.
It's now about 5 years or so since my cat died... my girlfriend and I have been living together for about 3 (almost 4) of those now and we just got two kittens together 5 months ago. They were only about 5 weeks old when we got them and we absolutely love them to bits.
It'll take time, but getting a new pet or pets really does help in the long run.