It seems obvious now, but it took me too long to realize I don't have issues focusing on things, I just have a hard time focusing on things I'm not interested in. I was a terrible student in grade school and took some time off after I graduated before going to university. That time away allowed me to mature and gain the discipline I needed to get through the stuff I wasn't interested in. I then breezed through my major because I loved it.

Don't spend as much energy on trying to improve the things that you're bad at. Figure out your strengths and lean into them.

If you have ADHD, get medicated. While it's no replacement for good habits, it makes starting those good habits SO much easier. I wish I had started taking it 15 years sooner.

If you also have ASD, a lot of people aren't going to understand you. The way you view the world is different. Be kind and be true to yourself and you'll meet lots of people who appreciate your difference.

Spending time being doing nothing actually helps your focus later, boredom is good for you and actually promotes creativity.

Don't multitask, you'll just end up half-assing every task.

If you have the means, see a psychologist who specializes working with people who are neurodivergent. It will greatly improve your communication skills. If you're in a serious relationship, get couples counseling even if things are going great. This will help you break down any communication barriers before they have the opportunity to become resentment.

I could keep adding to this list, but having an exercise routine is probably one of the best things I did to improve my focus. Plus I also get the long term health benefits of strength training.