Get good sleep. Ideally, you want 8 hours and a fixed bedtime every night. Sometimes this sucks because staying up all night can be fun but it throws off your rhythm and you'll be impacted for days. Once you get into a solid schedule and it starts feeling natural, you'll see how big a difference it can make. (This is also a part that can be very easily thrown out of wackwhack by meds.)
Eat breakfast. You want to have a good meal to start your day, your brain needs it to function. Don't eat cereal, you'll kickstart your metabolism but without getting muchmany nutrients and just end up hungry hours before lunch timelunchtime. I have had the best luck with foods high in protein and fat, like eggs and bacon and avocado for instance. You'll be full longer than usual and have more mental clarity.
Exercise regularly. Building up your body also builds up your mind. If you can run, swim, bike, lift, yoga, soccer, [insert exercise you enjoy here], this will help a lot. You need to do this regularly, I'd suggest 3-6 days a week, usually, you want to have a day off for body to recover. This burns off a lot of your excess energy. Exercising after work gives you something to look forward to, but can be hard to do when you're mentally tired from working and just want to chill. Exercise before work is hard to make yourself do because waking up early sucks, but you'll feel SO much better the whole day, and you start your day off right and feeling accomplished and proud right from the start.
Make calendar/lists/kanban for yourself. This has been mentioned by others already so I won't harp on it too much. You can experiment and find what style works best for you. I use a personal Trello board and have it broken down into Backlog, Doing Today, Current Task, and Done. Each task is color coded-coded based on how hard/time consuming it is. When you finish a task and drag it over to Done, it feels good. Nice little dopamine burst. Dopamine bursts will keep you going.
If you need to fidget, then fidget. If I am reading or sitting in a meeting I think best and retain information when I have something to do with my hands. There'sThere are lots of ways you can do this without being distracting to others. Twirl a pencil, get a fidget spinner (quietthe quiet one) or a fidget cube (these are awesome), silly putty, legos, something like that. I have a set of building block looking things (they're actually surplus equipment from our lab, but they look kind of sort of like legos) I keep in our meeting room and every morning huddle I make a different pattern or building with them. (Don't make noise!) My boss is aware of my situation and 100% cool with it, and even though I might not look like I'm paying attention, when it's my turn to participate in the conversation I can perfectly repeat everything that was said up to that point and then give my part too. Ironically, if I was sitting there motionless and trying to pay attention, I'd probably be thinking about video games and dinner and not mentally engaged at all.
Here's the cool thing though: you can harness this. Don't want to work on your big hard task? That's fine, you can procrastinate into lateral productivity. Quit working on your big hard thing, and do something easy and short. Maybe the easy short thing isn't the priority you "should" be working on, but you can take a breather from whatever is daunting to you and still make 100% use of your time. Then when you finish your big scary thing, woah, there's no more wounded soldier to finish off!
LastThe last thing I'll mention, look up How To ADHD on Youtube, it's a great channel. (I can't link it, it gets flagged as spam for some reason...? >:( ) While you're on that channel youyou should look into making your own calming/focus jar, it's helpful for a quick mental reset and also makes a neat desk item conversation starter. :)