Bad Day

This morning we woke up to the stiff still form of our feline friend Snookums. She had been struggling with some mystery pain the day before, meowing and dragging herself around trying to find comfort. I had sat by her side trying to massage her stomach, scratch her ears, anything to reduce her suffering. I thought she could pull through. And now I am riddled with guilt. Would taking her to a vet have saved her? There's no way to know. It was so sudden. She was off for the past week, but had been getting better. Then out of nowhere she was overcome by this pain. I don't know what brought her down. I wish I did, wish I could have answers. But the fact remains that she's gone.
The worst part is she wasn't our cat. We'd been looking after her long term while my friend was living with her aunt. We had her for four months, every day falling more in love with her. Now my heart is hurting and my spirits are low. Life can be so fleeting. And death is so permanent. 
I don't know how to feel.