1. Tell yourself, “I am now doing one thing and I will finish that one thing before starting another”
2. Make “finish” your new mantra.
3. Write down “I am now....” and do not move on to the next task until you have completed that task.
4. If you have OCD, these instructions will make no sense to you. Your issue is that it takes too long for things to feel finished. If you think you have ADD and you've gotten this far into the text without starting something else, you probably don't have ADD.
Remember the words of E.B. White “I arise in the morning torn between a desire to improve the world and a desire to enjoy the world. This makes it hard to plan the day.”.