If I've learned anything at all in life it's that the number one cause of stress, for me at least, is procrastination. Yes, there are other triggers, but when I'm really honest with myself I can see that most of the other triggers are a result of procrastination.
To illustrate, think about times when you've put something off until you can't put it off any longer, and how you felt when you finally dealt with it. Relieved?
However, the problem is that you don't suddenly begin to think of it when you are racing against a deadline. You think of it every day until you finally decide to get it done (while sometimes feeling resentful that the task has grown in scope -- due to procrastination). It creeps into your mind when you're driving, walking, doing other tasks, and even spending time with friends and family. It's like that extra five pounds you need to lose that won't let you fit into your favorite outfit; it's there every day and promotes a negative thought response that can bring down your mood and even cause you to lose sleep.
The good news is that this is an easy ailment to cure. Whatever it is that you need to do, do it now and stop making excuses. If the task can't be fully accomplished for reasons beyond your control, do what you can and communicate with others that may be involved. Don't leave it festering while using the excuse that "someone else needs to do their part." You can be the catalyst that pushes everything along.
“Things may come to those who wait but only the things left by those that hustle.” -- Abraham Lincoln
Making excuses, making promises to yourself that you don't keep, and projecting blame onto others are the stress-inducing side effects of procrastination, and they will not go away until the root cause of the problem is dealt with.
Focusing on doing what is yours to be done and doing it when it presents itself is an enormous relief and can be the path to freedom from daily, nagging concerns. Give it a try or make it a resolution, you might change your day to day life in meaningful ways.