I am in a similar situation.
Have a cat that is over 12 years old, came to us as a stray so don't know her exact age.
She has been a wonderful companion for those 12 years.
She was poisoned a year ago and suffered greatly but managed to pull through after a brave battle and lots of hugs and medication.
She is a shadow of herself now and obviously doesn't have long.
The vet says she has a growth now and needs surgery.
Deciding what to do is a heart breaker.
I wish you and your cat the best happiness that time can offer.