If you can’t make it, don’t say yes. It is important to know when to say no. When you know you can’t make it; the polite manner is to say no.
When you say yes, you are setting up the expectation, and when you don’t make it, everyone involved has to face the outcome.
And the most hurtful thing would be your own reputation. No matter how much you would convince, next time, your word won’t be taken seriously.
Yes, sometimes it is genuine that you can’t keep your word; at that point, do the restoration process. Say sorry and express that you understand how it might have impacted the other person and how it impacts you. Complete by assuring how you would make sure that it doesn’t happen again.
Make sure your restoration is once in a while and doesn’t become an every time process. Otherwise, you are putting your word again on the stake.
Do you keep your words? And when broken, do you make sure you restore your word?