I lost one to FIP. We knew there was nothing we could do, it was just a (painful) matter of time. So we took him to a park and let him play in the grass, poor guy was suffocating to death as fluid filled his lungs, but his last memories are chasing birds in a park under blue sky before going to sleep. We couldn't watch either, but I'll never forget that little guy with the biggest heart.
Very sorry for your loss man
Hang in there, you are lucky to have had 15 awesome years.