(This is a list of programs and tools that have helped others. This isn't an endorsement.)
Freedom distraction blocker
Time-management apps & other resources for ADHD adults
More time-management apps
toggl timetracker
Pomodoro technique
moosti timer
32 Tools and Apps Freelance Writers Swear By
books:
How to Organize Your Work & Your Life, Robert Moskowitz (I do like this book. It's got me where I am today, which may or may not be considered an endorsement.)
The Now Habit: A Strategic Program for Overcoming Procrastination & Enjoying Guilt Free Play, Neil Fiore