You will always overestimate the time you have to complete a project. This is law.
This is why your calendar is the most important tool you have to manage your projects and your life. Not your to-do list, not your GTD app. Your calendar keeps you honest.
It's your budget.
You may want (or even need) to complete 20 projects this year. But if your calendar only has enough room to complete 10, you're only completing 10.
You can accept it, work with your calendar and complete the 10 most important projects. Or, you can continue to fight your calendar, work on 20, and complete 5.
Put every project, task and event on your calendar today. After you pick yourself off the floor and come to the realization you can't do it all, start budgeting.
This part was a relief to me: I can't do it all. Just saying it out loud is relaxing.
Be honest. Budget your time. Do what's most important. Ruthlessly eliminate everything else. You'll be amazed how much time you spend on what doesn't matter. If you're like me, you'll also discover surprising truths about what does matter.