Your partner's mother put you in a terrible situation. It wasn't appropriate, and in order for you to move forward in any relationship with her, she needs to address it.
If you keep this secret, it has the potential to irrevocably damage your relationship with your partner. She may not feel she had anyone else to talk to about this.
Holding on to this secret also has the potential to create a toxic situation in your living environment. You can be sympathetic and loving, while also making it clear that a boundary has been crossed.
Talk to her privately and share what you shared in your letter: You don't feel comfortable keeping a secret from your partner, particularly this one.
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