Is it time?

My dog is 14 and a half, and I’ve had him since I was a few years old. He hasn’t been able to poop in now five days. I feel so selfish. I took him to two vets, they each independently mentioned his unknown heart murmur. He’s been urinating blood for a while. He hasn’t many tumors on his body, which we have operated on before and they came back. We did some tests but it’s still unknown if they’re cancerous. It might be a tumor preventing him from getting comfortable in the position and being able to poop, since the vet said it’s not an issue with the stool itself. He’s too old and weak to have surgery. We got pain meds. I feel like I’m just making him suffer. I know what needs to be done, I just physically can’t being myself to it. He still walks and drinks water and eats, but he was struggling these past few days and even today that he seemed better he was still definitely not comfortable. He can’t control his bladder. I go to school and would have to leave him with my family to care for him, but I don’t know how they’ll do and he doesn’t deserve any of this. I have beat myself up so much over doing what needs to be done. I can’t say goodbye. I know I have to. How can I put down a dog that is still walking and rolling in the grass? I know this is best for him and I want to be strong but I can’t.

Edit: I want something to tell me this is the right choice and that I’m not helping him by keeping him. I stupidly believe he’ll somehow heal and stay with me for the rest of my life. I don’t know how to let him go.