You can say no without attaching a whole TED Talk to it.

Long explanations are usually a request for permission in disguise. The more reasons you stack up, the more it sounds like a case somebody else gets to rule on, and the more material you hand over for arguing with. People rarely need your full schedule. They mostly need to know whether to count on you.

Try the short version this week: I cannot make it, hope it goes well. No backstory, no apology tour. If somebody pushes for more, calmly repeating the same sentence works better than inventing a bigger reason.