Probably because you're making it a joyless enterprise. You have high expectations, and you're making it a test of your worthiness as a person. So even starting

Probably because you're making it a joyless enterprise. You have high expectations, and you're making it a test of your worthiness as a person. So even starting to work on it reminds you of many personal regrets. You also have the problem that you're a very experienced Windows developer, so being a dummy again with a new topic is painful. Your body doesn't like feeling that way so it says "nope". I struggle with this problem too; I'm in a similar situation. But I think I broke my programmer's block by allowing myself to just do silly little projects. No big plans or expectations. Like, set yourself the task of rewriting a small program you did before. Or something small and personally valuable, even if it just gives you the scores of your favorite team or something. Think very small, low stakes.