Some tips:
1) Structure your environment to enable sustained attention. Be aggressive about turning off push notifications, unsubscribing from e-mail lists, leaving Slack channels that aren't relevant to your work, not accumulating a lot of browser tabs, and sitting somewhere out of reach of distractions.
2) Balance attention with activity breaks. Pomodoro timer apps are great for this. Start with 10 minute work intervals if you have to, but build discipline around respecting the timer. Stretch the work interval to 15 minutes once you've mastered 10. Then push to 20, 25, and 30 as you build up your focus. Treat it like going to the gym to get fit. Don't beat yourself up if you have to reduce your work intervals on tough weeks.
3) Be deliberate about time waster websites. Reddit, Twitter, Facebook, Instagram, and HN are okay in moderation, but you have to realize that each time you check these websites impulsively, you're worsening your focus abilities. They can't be your automatic mental escape when focus feels difficult. In the fitness analogy, these are like junk food. Okay as an occasional snack, but they'll take a toll if they make up too much of your screen time. Set aside time before work, at lunch, and after work for checking up on social media. Block them on your work computer.
4) Physical exercise. It's cliche, but it works. It doesn't have to be difficult. Taking a 10 minute walk around the building 2 or 3 times a day does wonders for organizing your thoughts and getting away from screens for a while.
5) Find supportive ADHD resources, avoid reinforcing resources. It's easy to find "woe is me" ADHD communities on Reddit and social media where people vent about their ADHD problems. Venting and empathizing with others feels good, but it reinforces all of the wrong behaviors. You need to focus your energy on resources that help you improve. Good resources include working with a therapist (check your company's insurance, probably cheaper than you think) and spending time around people who have high organizational skills.
6) Be accountable to other people. With ADHD, it's tempting to hide your progress and blend into the background. Avoid that temptation. Send weekly updates to your manager about your progress, without fail. Knowing that you need to show your progress every Friday provides some healthy pressure to focus and perform. Going back to the fitness analogy, consider this like being accountable to a gym partner. Accountability is good for you.
7) Eat healthier. This doesn't have to be difficult or expensive. Buy a bag of baby carrots and snack on that at your desk. Drink water, eliminate soda. Minimize sugars. It feels cliche, but it really does help.
8) Build your identity around being a software developer. Don't accidentally build an identity around being an ADHD person. Don't let your issues define you. The diagnosis is only helpful as a way to help you improve yourself. Don't let it become a crutch or an excuse for underperformance.