You usually pay your babysitter at the beginning of each month, but this month you forgot. She told you that you haven't paid yet. You say this about your mistake.
Oh, I'm so sorry. Thank you for reminding me.
You say "oh" when something surprises you, or when you hear new information that you didn't know before.
You say "I'm so sorry" when you did something wrong that caused a problem for other people. You usually use "I'm so sorry" when you're talking to people you don't know very well. With people who are close friends or family, you say "I'm so sorry" only for things you did that were really bad.
When people tell you about something important that you had forgotten, you say "Thank you for reminding me."
In a more casual situation, you can also say:
Thanks for reminding me.
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