I don't know if it helps, but this is not really a lie, and you shouldn't feel bad about saying it. You have your own reason for not being able to do something you committed to. Someone else might have a different reason that is equally personal that they don't want to share. "I forgot and I'm sorry" is a socially acceptable way to take responsibility without sharing specifics and potentially making someone else feel confusion or pity.
You can still work on the "why wasn't I able to do the thing I felt I needed to do" without worrying about "why wasn't I honest about my reason".
Sorry but the anxiety that accompanies my add says that your two cents are good in all cases but mine because my contacts will both know I'm lying and care deeply