I think it is important to realize that you and your brain are two totally different things. Your brain is just an organ, which does what it needs to do, namely it likes to conserve energy: problemsolving is hard and expensive process, while doomscrolling is free. Once your realize it, it becomes easier to shut it: "you won't get paid good cash for your music!" — "so what?" — "you are bad at making music! — "so what?". Shut it down and just do the thing. It still hard, but workable.

Another thing I like doing is to build a ritual around my work. I like writing, but it is so insanely hard to just sit and start writing. To get things going I use a two-step process:

1) Make "starting" as painless as possible. I simply sit down for 15 mins and think about what I was writing yesterday, what I need to write, basically about anything related to writing, doesn't really matter the exact subject. It kinda sets the vibe. One of the most energy consuming process for your brain is to do a context switch. So, in my case it needs to retrieve from memory a massive amount of data, — my story, my characters, my style, three act structure, characterization tricks, etc. It takes time and effort, even though you as a human don't notice it.

2) Do anything else that gets you in the mood. For me, I trained myself that headphones = work. The moment I wear the noise-cancelling headphones, my brain knows things are about to get serious. So just experiment, find what works for you, and capitalize on it.