My relationship anxiety has/had different roots, (no parental narcissism, I have a BPD dx) but I do understand to a certain degree.
It's not a cure all or a quick fix, but what I COMMITTED to do for myself and my relationship, was to just appreciate and enjoy the now.
That's it. Remain present.
Whenever I would get into a tizz about the future flaming ball of the wreckage that was to become my relationship, I would practice my breathing technique, deliberately set out to distract myself and just repeat "enjoy the here and now, tomorrow is not your problem yet"
Then I would send my bf an "I appreciate u" type message to cement my resolve.
It was by no means easy, first fee months it was actually damn hard... like I said, no quick fix. But the new "enjoy the present" thoughts soon became habit and I've not panicked, pulled or pushed away in a LONG time.
I took very long to become agreeable to the idea of therapy (eventually saw a T last year and am currently in DBT) so alot of my strategies (like this one) are self-taught. Mostly tips I've picked up here that I've tweaked to my own needs.
So FWIW, I hope you find this of some use to you.