Such a hard choice and it's always stinging whether or not it's for the better. We (my family) has had to put down 2 dogs and it's honestly so heartbreaking, one we had was about 15 and he started getting cancer and bleeding etc and we just couldn't take care of him. And the other was a case of epilepsy that it was so heartbreaking to witness and we just couldn't manage the payment for treatment and medication let alone seeing the poor guy go through with the ordeal, that dog was honestly the best dog we've had (out of 4 so far.) But, all and all I feel your pain and there's always more guys/gals out there to adopt and give the best possible life for them you can. Even though they may not match him they will be forever grateful in the chance you give them.