I have two things to suggest. I hope they help. If I'm being blunt, I apologise.
1. Simplify your thinking.
If you have a habit of overly abstract thought, you can make problems more complicated than they need to be. You could say it's a bit like spiritual java programming.
For instance, aims such as "become a virtuoso musician" don't mean anything. They lack concreteness. "become ascetic monk on mountain top". Also intangible.
You say you recognise them as daydreaming. But your whole way of thinking is daydreaming - "What I think ..", "What I fear ...", "doom", "failure". All abstract daydreaming. Do something about it. Stop daydreaming.
Make a promise to yourself: you will reject thoughts that don't have a tangible aspect you can act on immediately. They are tiring you. Let your brain rest awhile. And if you have to think about problems, think about other peoples' problems not your own.
That means real, tangible people. It could be anyone. This is not a moral injunction. It's practical - if you look to see how you can help other people it requires empathy and if you develop empathy you start to see other patterns of thought. It may help you see different ways of thinking and get you out of your rut.
Only think a few days in advance. Reject all thoughts of the future for a while.
2. Tone down the ambition.
You imagine a virtuouso musician or some epic bike journey, some massive startup. Therefore anything you actually you do seems crap in comparison - attempts noodling on the piano or going for a small bike ride around the neighbourhood seem silly. Trivial. Pointless. Why bother? Stop comparing yourself and your life to your daydreams (see point 1).
I write this because this is what helped me. Please ignore it if it doesn't feel right.
Best of luck, I wish you all the best.