“I've changed my mind” can sound like an admission of a mistake. But the reason may be different: we learned something new, understood how much effort it would take, or realized that our first yes was a way to avoid an uncomfortable conversation.
There are different ways to change a decision, though. Honesty does not erase responsibility for promises and their consequences. If someone was counting on us, it matters not to disappear silently: tell them as early as possible, explain what changed and discuss how you can help. Pretending the old answer still works does not make the situation more dependable either.
You can start without a long defense: “I need to change my decision. I can't take this on. I'm sorry to change our agreement; can we talk about the best way forward?” The other person may be upset. But now they have clarity, and we have the chance to stand behind an actual decision rather than an automatic “of course.”
Changing your mind is not always carelessness. Sometimes it means acknowledging that your earlier answer is no longer enough.
The stars speak. The choice is yours.