That's going to be wonderful, a true momento of someone who means the world to you. I hope he turns out perfect!
I understand your pain, I had a Border Collie named Sonny who I'd had since I was about 12, he was my sister's dog for a long, while, but he liked to hang out with me, then my sister moved away and he basically became my dog. He slept on my bed, he stayed next to me all the time, I took him for trips in the car with me, he was awesome. Eventually he developed a tumor and within a matter of a couple of weeks he was too weak to move. We decided we would take him to the vet one more time to see if there was anything else we could so treat it or...if it was time...I hated that day, so much. I went through to my room to grab my car keys, license, etc. and when I came back, he'd passed. It crushed me. I was inconsolable for days, I still get upset thinking about it. I felt selfish for letting him suffer, for wanting him with me as long as possible, and yet I also felt devastated that I wasn't there at the very end, I'd walked away for a couple of minutes and it was over. It's such a horrible, horrible thing to lose a part of your life like that, and it hurts for a long time. Know that he loved you, and no, you didn't cause this, it's not your fault. I completely understand the guilt you feel for not being with him at the time he passed, I do, I felt that same guilt, but it's not your fault. I know it's hard to see it right now, because the loss is so fresh, but take the time to grieve and in time you will let go of that guilt, because this wasn't your fault. My inbox is wide open if you want to talk!
I don't think he felt betrayed, at all. He loved you. You came back for him, kept him close and gave him a good life. He won't have had any resentment towards you, not at all. This wasn't a betrayal, either. You were doing your best to get him the help he needed, that's an act of love, not betrayal.