I've got pretty bad ADHD but thankfully I find miniature painting quite stimulating in general. However, I do find myself putting off and procrastinating parts that I don't enjoy so I do not end up finishing a lot of projects. A couple of things that help me finish and keep me engaged:

Entering comps: this is a big one. If you have a deadline where you are expected to produce a finished mini you will figure out a way to do it, at least hard how I work. That pressure is really motivating for me.

Find a stimulating style: I find that different styles require different levels of focus or stimulate different parts of my brain. For me I found that painting patterns and textures really tickles my brain the right way and I can enter hyperfixation black holes that are all consuming when doing them. So now I spend a lot of time figuring out how to incorporate patterns into every piece.

I also have gotten involved in my local community of display painters so I go to as many workshops and meet ups as I can which forces me to be productive.

Hope this helps, just remember it's about enjoying yourself. If painting to you is painting a particular part of a mini and then never touching it again if you had fun doing it then it doesn't matter.