How do you know when it’s the right time to put your pet down?

My dog is 13 and has heart disease. He’s been on medication for a while. He is losing his appetite and throwing up a lot of fluid. He is sometimes struggling with breathing. Last week he had two decent days but the rest were not good. I’m often laying with him in his dog bed rubbing his back which seems to help him sleep at night. Today the vet examined him and changed the doseage of his medication and told me he really doesn’t feel well. We will talk tomorrow/Wednesday about putting him down

He is my baby. I don’t want him to suffer. I need to make sure I’m doing the right thing