You act a bit indifferent about the relationship. For example my last gf, she said, "you are TOO cool about this". Because I was constantly unphased unjealous and open to her ideas of it ending in certain scenarios. Like I was never saying yes I will bend to that or I will promise the world. I just was like, oh yeah if xyz happened I could see why you would want to go elsewhere. Constant acceptance of the stuff other guys fears will make her fear you are TOTALLY unphased and prepared to land on your feet like a cat.
She was an amazing girl....
That said, future scenarios should not scare you. Most guys are very concerned about them. So the girl feels she has him, but if u fear nothing a person has to wonder. It is not something to overdo and is only part of what you want to do, but if you go more that direction u will have it
I would add you never put the relationship over your own needs or development. You should contribute to the relationship of course, but too many guys make themselves slaves to the relationship. At the end of the day, the only person you spend 24/7 with is you. So if you're not happy with you, no relationship will ever replace that. So you have to keep fearlessly developing yourself and pursuing your goals. I've always had girls feel like they're "at my heels" as I'm creating a powerful reality in my life. When they feel like they have to keep up with your progress, your drive...then they're always chasing.
Also, I've told every girl I've dated in my adult life "You will never be the most important person in my life." Even if they are number 1, which is fine, they can't be the ONLY number 1. What does that say about my family? Friends I've known for half my life? My great colleagues? Why would a girl I've been seeing for 6 months suddenly take precedence? Life is a latticework. You need a village to thrive, despite what Disney might try to tell you