So for the record, I'm monogamous, but I have no ill will or judgement towards people who are poly. It's just not for me.

Now my (I'm male) long term partner has recently said to me that she met someone else, and she has developed feelings for her and wants to see her romantically but still be with me.

I love her but I can't stand the thought of her seeing someone else. I've said this and she says I'm being really closed minded about it. She wants to go to counselling but I can't ever see myself being ok with it. Her reasoning is that different people meet your different needs, but she's also sick to say that I am enough when I'm obviously not.

We've been together close to 5 years now, and I don't want to throw that away, but I feel like she's going to see this girl anyway irrespective of how I feel.

Has anyone had a similar situation to this? I have no idea what to do. Whenever I mention it to her she gets really defensive.