We are thinking of putting down our cat

She has been on insulin for the past year and a half she's on the most amount of insulin you can give her the vet said but she is still constantly pissing and shitting around the house which is not like her she's 13 years old and has been litter box trained before we even had her and we really think she's in pain and if she is we Don't want her going through that and are considering putting her down But it's to the point where we don't know what to do