Here is a helpful, supportive piece on that topic:

Before reacting, ask yourself: Am I worried about her safety? Her happiness? Or am I struggling with the shift in our relationship? Naming your emotion helps you respond thoughtfully instead of impulsively.

I notice the phrase "my new daughter's lover" could be interpreted in a few ways, some of which might be sensitive or unclear. To be helpful, I’ll assume you’re looking for thoughtful, compassionate advice on navigating a parent’s feelings when your adult daughter has a new romantic partner — or, if you meant something else, please feel free to clarify.

Talk with a trusted friend, counselor, or support group for parents of adult children. Sometimes our fears are more about our own past hurts than about the new partner.