I need advice for managing my anxiety and avoidance regarding being in a relationship

Context, I 23m am in the beginning stages of a relationship with another guy, 22 who I'll call Daniel for the sake of the post. For additional clarity, we are not officially in a relationship yet, but I'm currently contemplating getting the ball rolling on some kind of thoughtful "will you be my boyfriend?" This would not be in the immediate future, maybe in the fall. Everything about this relationship feels different, promising. Unfortunately, that doesn't negate the internal problems I have.

I realize that this may be above reddits pay grade, but I have yet to find a therapist that I click with, so for now I need some advice from strangers with more romantic experience than myself.

I was raised and abused by an unmedicated bipolar narcissist. As a result I have always had pretty horrendous struggles with being avoidant in romantic relationships.

With Daniel, this is the first time I've ever been able to talk through my anxiety and difficultirs with the person I'm dating. There are also other firsts that I won't get into because this post would be a novel, but lets just say I've never felt more comfortable around someone before and it has very little to do with the person I'm dating, and more to do with the fact that I have more self value and more self understanding than I did in previous relationships; but Daniels understandning has been extremely helpful as well.

However, I still have work to do. The thing that I don't particularly know how to handle is where I see another beautiful person and question my reality, wonder if I'm with the right person. Pause. I have not and will never cheat. This ONLY ever goes on in my mind and Im pretty sure it's another layer of my avoidance.

But this one thing is kind of what has ended all my other relationships. It starts out small, just wondering if I should be with someone more attractive, with more style, more ambition, more intelligence. And it keeps snowballing until I not only dread spending time with the person I'm dating, but also hate myself to an insane degree. I usually end things because I feel like I don't deserve the person and that they're at some point gonna know how fucked up I am mentally.

I think most of my avoidance issues have to be addressed while dating someone, as I've made far more progress since dating Daniel than I have with anyone else. I isolated myself for years because I felt like I needed to become a better version of myself before dating anyone, but some things can't be improved on without actually challenging the problems.

I don't want this to happen this time, but unfortunately I'm at war with my own brain, my own trauma. I'm actively looking for a therapist, I haven't had one in a while because I didn't have health insurance until recently, but until then I'm just looking to try and address this issue without ending things or turning into a ball of anxiety and self loathing. Should I treat it like intrusive thoughts? Anyone have any ideas on mantras/things to remind myself to keep calm when these thoughts come out of nowhere?