Is it time to put my pet down?

I have a 13 year old bearded dragon who has had a lifelong heart condition since he was a hatchling (he even looked sick as a hatchling). He’s to the point where he can’t walk more than a few paces before getting out of breath and if he moves too fast has fainting spells. He is still eating, scratches at the glass when he wants to be let out and is going about his daily activities as usual, just really slowly. The vet says there is nothing we can do for him, but that I shouldn’t put him down yet since he still eats etc. I just don’t know what’s right. He is suffering daily. He seems to still have the will to live but how can I tell if it’s just survival instinct?