• don't worry about maybes, enjoy what is while it is.
• take things slow if you're very concerned about it being a problem.
• communicate your concerns and fears with her in a non-confrontational manner.
• keep in mind: no one will leave for someone else's best qualities, rather they maybe find someone to help escape your worst qualities.
Be the most healthy, attractive, smart, well-rested person you can be, always. That's the one tactic that will work the best.
How can you be sure that your girlfriend/wife will not leave you for a richer and more handsome man if the opportunity arises, even though she claims to love you?