It might be that drugs will help here, but even if you’re on drugs, I think brain training over long periods of time is worth investing in. Some examples which I have put effort into:
Mindful meditation. Every time your brain drifts, notice it, and correct it. Practice until you’re good at it. It will take years.
Brute force reading. Sit down to read something you know you need to read, but that you know you’ll have a hard time with. Every time your brain drifts, notice it, and correct it. If you can’t remember what you just read, go back to the top and read it again until you do. Practice until you’re good at it. It will take years.
[If you’re in a cover band] Play boring yet incredibly popular songs with your band. Every time your brain drifts, you’ll notice it, because you’ll forget where you are in the song and you’ll make a mistake and your band mates and the audience will notice. It’s brain training for focus, with an actual social consequence.
Oh yeah. A wetsuit helps me immensely as well—I just lose heat too fast otherwise. It turns a chilly experience where I have to keep moving all the time, into a nice relaxing thing.