If I say something that is true in the moment, then a minute later realize it’s not true; but I don’t correct myself because I’d rather they believe what I said than the truth: Am I lying to them?
If your intent is to deceive, then yes, it’s a lie.
The extent to which we should do something depends how important it was.
We can always remind ourseves to slow down next time.
If you decide to let it go because it’s not important, that’s one thing (e.g. you said you went to the dentist on Thursday when you really went on Tuesday). But if you are trying to deceive, then I would say it’s a lie.