Sometimes a delay is better than doing a halfway job. This is both a valid marriage lesson and why you are not reading the post you expected today. I had one of those days, and the promised post is too important (and too involved) for me to do given how exhausted I am now.
If you regularly have to back out on what you have said you would do, you are over promising and you need to figure out how to change things. On the other hand, life happens, and sometimes you really do end up short despite your best plans and intentions. A simple “I’m sorry” and letting her know how you will make it up to her is better than doing a halfway job and passing it off as adequate.