There is going to come a time where you miss a deadline. It will suck. It will haunt you. It will make you feel like a failure as a writer and planner both. It will poison your confidence and make you consider quitting the biz entirely. Let it settle in. Roll around in it for a few hours. Then get back to work.
Missing a deadline is not the end of the world. It is also not acceptable behavior. The key in these situations is to recognize how you got into this situation in the first place and learn from that so it doesn’t happen again. If it does happen again then you need to consider how much you are taking on as well as your willingness to put aside other things to get the work done. A writer’s life needs to be about writing. That needs to happen most if not every day. You need to find ways to be productive, even if that means writing outlines and adding a paragraph here and there to help you get what you are working on in better shape or clarify your ideas. You cannot decide that you’re going to blow off work and do other stuff primarily, because that is how you end up backed against a deadline in the first place.
Here’s another thing about deadlines they don’t tell you: When you are racing to write up to the deadline or especially past it, you aren’t writing your best, most relaxed, and thoughtful work. You’re writing the crap you forced onto the paper to make the wordcount. Don’t write crap to make wordcount.
You’re better than that.