Had to put my Puppy down because the Surgery was too expensive

I just had to put my dog down an hour ago, but I don't know if I could've done more. He has been sick since he was born and only made it to 6 months. We didn't know he was as sickly as he was until he stopped eating, drinking, and peeing himself 3 months previous. We took him to the vet, and unbeknownst to us, it was just a $1700 bandaid over the issue. We got his meds and fed him 4 times a day to get his weight up and Pedialyte for vitamins. We thought he was getting better, but this morning he woke up and did the same thing he did 3 months previous, but this time, he seemed worse. He wasn't moving and barely breathing. I took him to the vet, and they told me he was extremely sick and needed extensive surgery that would cost $4-6k, and it still wouldn't tell us exactly what was wrong with him. The vet told us it was more than likely a genetic issue because of how long he had been sick. I had no money left to spare for his surgery as this new year has been horrible; I had to fix my car which was $1400, still paying off his previous treatment, and my college tuition is $3000; I have no more money. He wasn't going to make it to the end of the day if he didn't get the surgery. I called my local Humane Society, and they wouldn't take him either so the Vet recommended euthanasia because of how much he was suffering. I just didn't want him to hurt anymore so I think I did what was best. I think I did. He was a baby.

I hope this makes sense as I'm typing this through tears.