My girlfriend and I have been together for more than a year. She told me that she wants a polysexual relationship because she feels something is missing from ours. Every time I say that I don’t want this, she gets upset. I love her and she loves me, but she also loves another person and he loves her back. She said that the only reason she loves this other person is because he reminds her of me. I want it to just be her and me. What should I/we do?

Tell her to make up her mind. She may be genuinely polyamorous, but you clearly are not – so this situation is untenable.

It would be better to be clear about this now and avoid prolonged pain. Give her a choice: either monogamy with you or both of you move on. I don’t know your age, but I suspect you are both pretty young. People under 25 are commonly still searching for a sexual and relationship style that fits them, and many want to try a number of modes in order to inform their decision. So, it may be that your girlfriend is testing the notion of polyamory and might withdraw her request if you set boundaries.

Alternatively, she may be trying to find a way to move on without hurting you. Call her bluff. In clear language, say: “I really care about you, but you are asking for a relationship style that won’t work for me. I do not want to share you. Please decide if you want to be an exclusive couple with me or not. If not, it’s over.”

• Pamela Stephenson Connolly is a US-based psychotherapist who specialises in treating sexual disorders.

• If you would like advice from Pamela Stephenson Connolly on sexual matters, send us a brief description of your concerns to private.lives@theguardian.com (please don’t send attachments). Submissions are subject to our terms and conditions: see gu.com/letters-terms