How should I feel and handle this?

My ex just told me that he's in love with his best friend from high school who he went out with back in high school. But the dilemma is that she's married now with a child. She also has feelings for him and her marriage is on the rocks.

This honestly broke my heart a bit. It's like the more I talk to him, the worse things get for me. I just want to end communication but I don't know how. I thought we could be friends but eventually I know he's going to date someone else whether it's his best friend or someone new and it stings a bit.

I know you guys warned me of this happening, but I just thought I would be able to handle it. I thought a friendship with him would be easy now that we've both moved on.
How should I feel and handle this?
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