It's not fair to you and has clearly compounded your pain. Decide for yourself that whatever it was, it wasn't important enough for your mother to mention.
Tell her: I miss the closeness of our old relationship, and I need to get it back. What can we do to repair this? If it takes me never asking about the secret again, I'm willing to do it.
Over time, your aunt needs to apologize for creating this turmoil. I hope she's able to get over whatever she's built up around herself and be there for you.
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