My cat died after having her for 15 years( If you feel the need to say a nasty comment, go elsewhere)?

I have had this cat since I was 10 years old. I am now 25 so it has been pretty rough the last couple of days. She died this past Sunday. I don't know how to cope and I have taken an emotional blow. I feel guilty because I am away on school and I feel that I abandoned her-she was living with my parents so she is much of their cat as she is mine. Please don't say she is easy to replace ( I don't see living things as commodities). I'm not sure what to do but I did see it coming although she didn't have a chronic disease. She died peacefully in her sleep in her favorite resting place curled in a fetal position. Every time I have dealt with death, it lingered a bit before it struck. I think the hardest part is remembering her when she was young & full of life and then watching her age to a withered being before my eyes.