I've thought this for years. I think he lost interest in finishing the books because he'd accomplished his goal by the time HBO picked up the series. He had created an entire world that would live on after he was gone. That's his legacy. Once he hit that critical mass, he lost interest in completing the story. I also think that's why he intends for his papers and notes to be destroyed if he dies before the books are finished. At first I thought that decision was horrible, then I decided he intended to give the following writer a free hand. But now I realize it's not really his specific plots that matter to Martin so much as the existence of the world itself. Other writers will inevitably carry on, but for him, it's all about the world building. That's why he's writing the prequel (plus he's getting paid a large sum by HBO to do it). This way, he gets to build out the world some more without cluttering the original story with more characters - a real problem that began with the fourth book.
Meanwhile, Martin says life is too short to spend so much time sitting and writing, and Gaiman says George is not our bitch, etc. Okay, fine. But the fact is, Martin's butt is happily in the chair with hands on keyboard quite a lot, just like when he edits all those Wild Cards books. Clearly, he works on what he's interested in. That's my theory anway.
So here we are. Martin gave HBO the big ending when it was apparent they were pulling ahead with the understanding that they would get there with their own plotting. Now he's complaing about some of HBO's character decisions and thinking out loud that he kinda probably maybe should've finished the books first. Seriously, I feel like he screwed over the original readers. I would rather finish the series before watching the show, but the ending will be spoiled by a public who won't shut up about this pop culture event. So, I have zero sympathy for Martin and his career choices.
But he is a good writer, and I'm looking forward to reading Fever Dream.