For overthinking, a CBT self help book actually helped me to analyze that mess that is my brain and see lots of pointless worries and anxiety and emotions. Been a horrible overthinker most of my life.
Thoughts, emotions and behaviors form a kind of feedback loop and overthinking is often fueled by anxiety, at least in my experience. Thinking of all the ways things can go wrong or you can mess up. I guess there's some amount of this we can't get rid of with ADHD, because of being forgetful and inattentive etc., but there's lots of worries that are blown out of proportion, like what other people could think, and always elaborately thinking about the worst case etc. So that's stuff one can try to realize.
What helped me was realizing all the stupid stuff I think about and look at myself from a distance and then kind of realize how ridiculous it is. Thinking too much about what happened and whether what I did or said was ok, or worrying about something that can happen in the future. Other people have their own lives an worries and do not think as much about us as we do, and there are many things outside of our control, also no point worrying too much about those, etc.
You can try to create a mind map of what things you believe about yourself and others and why and follow thought spirals, and then looking at this crazy mess of a map shows a lot of garbage. Just have to be critical of your own reasoning and feelings, there's a saying "don't believe everything you think". Truly internalizing this really helped me getting out of it, I believe.
Concerning motivation... Well, I think getting meds was the most impactful thing, helps me a lot to do and complete stuff that I do not enjoy, and be more calm (less thoughts, less emotional swings). But changing the attitude to certain things does also help (like, I'm not doing chores just because, but e.g. because it makes my wife happy and reduces her work load, and I want to see my wife happy, etc.)
Nobody knows yourself better than you do, so nobody can cut through the bullshit or find tricks for your brain better.
Hope this helps a little bit.
Good luck on your own journey :)