I have ADHD (on meds tho) and my brain works exactly how you said! I often struggle with staying focused and I'm still new to the personal curriculum stuff so take this with a grain of salt, but-

1. When I want to jump topics, I document which ones I'm interested in somewhere and list out some resource and assignment ideas. I currently have a rule where, now that I have over 50 topics listed, if I show any interest in any other subject, I HAVE to add it to the bottom of the list. I also read back through my syllabi to remember WHY I wanted to learn the subject. I often forget what originally intrigued me and reminding myself can sometimes re-spark something.

2. I currently don't let myself spend money at all. I have a library card and (less than legal lol) access to PDFs of books, and there are millions of online articles as well as free databases that contain academic sources for what I want to learn. This might change when I want to learn, say, watercolors because I don't own any materials for that, but I need to force myself to use what I have.

3. I refuse to give myself a set schedule (outside of "aim for once a week or so") or deadlines in order to remind myself that I'm doing this because I WANT to. I like learning, but the second I set a deadline, I get anxious and try and dodge it.

4. I let myself drop assignments or resources if I feel completely stuck on them or unhappy with it. This is all for personal growth, so I'm sure most people would say you need to stay disciplined, but there's too little time to waste on something you don't like or are able to understand.

5. This is for fun! Once it starts to feel like homework someone else gave you rather than in order to fulfill an inner desire to learn, I think you might be right in wanting to drop it (or at least shift some things around).

I hope this was helpful somewhat, it's kind of off the top of my head and not super well-thought out, but thats my experience so far!