If your eyes are wandering toward the mother, it usually means there is a "spark" missing in your primary relationship. Work on reigniting the chemistry with your girlfriend. Focus on the things she has that the mother doesn't —their shared history, her specific personality, and your future together.
If this is the case, end the current relationship gently and then pursue age-gap dating on apps or in social scenes where mature singles congregate. There are millions of fine women over 40 who would love a younger, attentive man – and none of them are your girlfriend’s mom.
Accept the thought as a fleeting observation of "good genes" rather than a romantic directive. Focus on the Unique: