It's a perfect relationship.

[Sepher]

Why does the fact she's been with others previously and decided she no longer wants to be with them make you feel so insecure? Simply turn it on its head realising that of all the previous lovers she's had right now she chooses to be with you in preference to any one of them? That could be securing and comforting and a source of pride and deep contentment, not a cause of insecurity, petty jealousy, resentment or threats to your ego. You could even explore why it was she choose not to be with them, and what so far makes you different, what was good about their sex lives, and what wasn't, and what's good about yours, and what could be improved upon? It could be really self-affirming and validating if you only switched your thinking around.