I am a passionate self-learner and chaotic multitasker. I am currently doing exactly what you mentioned, learning a language and exercising, among a full time job, and having two children solo every two weeks.
I don't have the same method for both: for exercise I have to just stick to plan and show up even when I'm tired just because it's a health matter and I am experiencing that it's positive for me to an external (aesthetic) sense and to an internal (well-being and psychological) sense. There are times in which I fail to stick to the plan, and I learned I have to just keep restarting.
While with the language learning I am now very structured, I attend a course regularly, do drills for vocabulary, ask people about their culture and music and stuff so that I am exposed to it. It's much more structured than my approach to exercise - maybe also because it kind of turned into my hyper focus, haha.
What unites both methods to me, though, is consistency. Showing up. Being there. Trying, even when I feel ridiculous declining everything wrong in my new language. Accepting to be like a child for a while.
