Sorry for your loss. I went thru this twice. One cat I chose to try all the meds and she died anyway then I felt guilty for having made her suffer. The other one I chose to end his suffering after having his lungs drained (they filled right back up). I felt guilty for him too even tho the vet said it was time and nothing else they could do. They were both only 3 or 4 yrs old.
It's a damned if you do damned if ya don't situation. Try not to be too hard on yourself.