What realization does Lambert Strether reach about his own life in The Ambassadors?

In Henry James's 1903 novel The Ambassadors, the protagonist Lambert Strether undergoes a profound personal transformation after being sent to Paris to retrieve a young man.

As he experiences the aesthetic and social richness of European life, Strether reaches a poignant and sad realization that he has spent his own life unwisely. He comes to understand that he failed to embrace his youth and missed opportunities to experience the world fully. This epiphany leads him to urge others to "Live all you can; it’s a mistake not to," a declaration that serves as a central theme of the work.1

Ultimately, Strether resolves to abandon his role as an "ambassador" for others, choosing to reclaim his own agency. However, the novel maintains a tragic perspective, portraying Strether as a figure who, despite finally choosing his own destiny, remains constrained by social regulation and a sense of having lost his youth.1