What do you do when you cannot meet a deadline? You're getting close to the due date and you know it won't happen "on time".
We usually fix goals' deadlines arbitrary without knowing in advance how long it will take. We just hope to get it done by a certain date so we can make progress, improve a milestone, launch a new product, get our performance bonus...
Most of the time those deadlines don't matter.
Unless it's bound to a specific event which you cannot postpone. In that case you need to minimize the negative effects it will have on that event, and worked out the best solutions with the remaining time you have.
Ask for help.
Don't wait the very last minute to tell your boss, colleague or partner. Anticipate as much as you can to find solutions. Missing a deadline can be bad, but not telling or acting on it soon enough is even more damaging.
Reflect & rearrange your work.
Reflect on your progress, on how much you were able to get done in X amount of time.
Did you take the right daily actions to make it happen? Was this goal really a priority or other things got in the way?
Reset your next deadline, work out how you're going to keep working on that goal or project over the next month or couple of months.
Deadlines are not always efficient.
Know that all projects or goals don't necessary need a due date. Some just need to be prioritize and get done asap. If you decide you gonna achieve a goal over 3 months, it will take you 3, more or less. If you decide to work on it for a month, or asap, you might actually get it done in 3 to 4 weeks or less. Focus on short time frame and make sure you work on your goal each day at least 30 min to 1 hour, that is if you cannot focus most of your time on it!