Boy, can I ever relate. My cat's vet recommended we have my precious cat put to sleep but we kept her an additional 3 weeks (unable to make that decision). In those 3 weeks, I witnessed her suffer so very much that it was almost a relief when I called to make the appt.
I was 8.5 months pregnant at the time & was so physcially ill when we brought kitty in for the last time, that I vomited for 3 days. Almost had to be hospitalized. We had my little "Mishetta" for 15 years. They become your children, so I really do understand your pain. Just do it, if it will mean peace for your little fur~baby.
Bring plenty of kleenex with you & then come back to us so we can try to {{{{hug}}}}} your pain away.
God Bless!