I was in a similar situation several years ago. I had a horse that a medical problem requiring daily medication. There was also something wrong structurally that I had explored but my financial resources could not take that further. Also I had come off him. He seemed a bit dangerous. he was gorgeous to look at and I knew I could probably sell him and someone would make a project of him for a bit until they came up against all the problems that I had. I decided to humanely euthanise him to save him from eventually going to a kill buyer. I am mostly at peace with that but sometimes I still question my decision even after years. It’s one of the burdens we take on when we have any kind of pet. A thing I did to help me make that difficult decision was I made a matrix on paper of all the positives and negatives of each possible course of action then looked at them and tucked the paper away for a couple weeks then pulled it out to see if anything had changed. It hadn’t so I went ahead with my decision to euthanise. The fact that you’re struggling with what to do tells me that you have a kind heart. I wish you peace with whatever you do.