Schedule an appointment with your vet just for asking questions and exploring options, and ask about a good time for phone consultations. Share your feelings and worries with people who understand the relationship you have with your pet. When you must make a decision about care, ask trusted friends or family members for opinions. If, on the other hand, they seem to need some distance, don't force their involvement. Think about quality of life.
Individual animals have different personalities and levels of tolerance.
You know your pet best, so you're the expert on what keeps him happy day to day. It may help to write down your thoughts on this. Suffering is harder to evaluate and may arise, for example, from a greatly diminished ability to engage in daily routines or to interact with you. Remember, as you explore this difficult issue, that there are no right and wrong answers. Take your time, and trust your instincts.