When is it the right decision to euthanize?

Tagged NSFW just in case talk gets a little graphic.

I've had my beardie for 13 going on 14 years. He used to be so healthy, but in the past 4 years, I have to manually open his mouth to feed and water him. He still did normal beardie things, but slowly went downhill. Where I lived had no vets within 3 hours that took care of lizards so I scoured forums, tried all sorts of vitamins/supplements, saved and got all sorts of temperature controls, automatic lights, humidity gauges, top of the line gut loading food for my feeder insects, etc. I've finally moved to a bigger city and there are vets that do happen to care for reptiles. Unfortunately, he doesn't try to move much, barely wants to open his eyes, he won't let me manually feed him, and doesn't seem to enjoy playing or being out of his cage. One of his favorite things used to be chasing a spoon, but now he just closes his eyes and doesn't care.

Is this the point where I have to stop being selfish and let him cross the rainbow bridge? Thanks for being here for me.