Putting limits on his relationship with her is not the way to go here. Instead, focus on what you need. If it is a dedicated date night for the two of you, then do that. But telling him he can't see her isn't fair to either of them. She may need 2 or 3 dates a week to feel connected, while maybe you would be ok with only 1. Don't compare what she is getting with what you are getting - instead focus on your needs and go from there.
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