How can I be secure in my relationship with being my girlfriends first everything?

I've been treated horribly by most women I've tried to date, cheated on, ghosted, put down for no reason, and mostly ignored by the majority, I'm my girlfriend's first everything, we've been together almost 3 years but I can't help but feel like she'll eventually want to find another guy since I'm only her first and usually everyone has multiple partners
How can I be secure in my relationship with being my girlfriends first everything?
Post Opinion