IWTL how to stop believing that every partner will eventually leave me for someone else

This feels like my white whale. I have worked through so much in the passed seven years and come so far, but this issue is the one I think I have been waiting to face down. I know it has everything to do with my own feelings of worthiness and fighting my executive function issues (I am so afraid of not being able to pull my own weight in a domestic setting and that ruining things) but I really struggle to shake the feeling that a partner would stick around just because they love me.

Realistically, I know that being in a relationship is a choice you make every day and that ultimately there will always be someone prettier, smarter, more successful or unique out there that your partner will cross paths with. However, I really struggle to believe that a partner would still choose me in the face of meeting so many other options, like their curiosity might be piqued and lead them to befriend those people until eventually they realize they like someone else more. I have only recently realized that i have been subconsciously holding this belief as an inevitability, not just a possibility.

Part of it is that I don't think I've ever had a relationship where a partner loved me in a way where they weren't expressing interest in others or being noncommittal to me. I'm in my 30s and never had a relationship that lasted a full three years because of my unconsciously seeking out emotionally unavailable or noncommital partners. I have at least stopped doing that and am not currently dating at all while I work on myself, but I want to learn how people believe their partners when they claim to be committed to them and how to trust someone to be committed to me until their actions prove otherwise. I don't want to spend my life and relationships driving good people away because I'm looking around every corner for potential "better options", waiting for the other shoe to drop, and constantly trying to prove my worth to partners regardless of how well they treat me.