This is a question about rebuilding trust after it's been lost

Posting here because me and my nesting/anchor partner are polyam and this issue is about poly things
Everywhere else I go just talks about cheating or similar monogamous problems

Me and my Anchor/Nesting partner have been together for nearly 3 years
This issue has been happening for about the last year

The issue stems from how I handle New Relationship Energy (NRE), that burst of postive fluffy emotions that come with a fresh new relationship

Im AuADHD so In the past Ive had a tendency to fixate or hyperfocus on new people
Making my Anchor Partner (lets call them partner A) feel neglected

This has happened twice now

After the most recent, and most egregious instance, i sat down and tried to reevaluate how I handle new people and new relationships
I needed to slow down, think abt all the details, consider everyone not just one person at time, etc. etc.

I made a new system for going about new people
I re-wrote the code so to speak
(Being AuADHD that metaphor is pretty literal lol)
(I also realized i needed to raise my standards for people that i start romantic/sexual relationships with)

After I had that reset I was genuinely hoping for, though not necessarily seeking out, a new relationship
I wanted to see if my new mindset/method would work
Me and Partner A had a running joke that "I need the practice"

Well 2 months ago I got that chance
Someone came up to me in a cafe, we got to talking, and we hit it off immediately
They are intelligent, cute, and a fantastic cuddler
I can honestly say that ive fallen in love with them

But more importantly
The new code worked.
Every step of our relationship has been deliberate and measured
We didnt rush into anything
I didnt fixate
I didnt neglect partner A
And im pleased to say that i now have a Partner B

Weve been together for a month now and i couldnt be happier with how things are going

but

Partner A still doesnt trust it
they told me they feel like thier holding their breath waiting for something to go wrong
they want me to make an effort to activley rebuild the trust in our relationship
but im not sure what to do or where to start

in my mind fixing the issue, 're-writing the code', is about as active a solution as is possible
i changed my system, tested it, it worked perfectly
but to them simply 'not doing the thing' isnt enough

Partner A does absolutley want me to be happy with Partner B, they are happy that i found someone i care so much about, they would love to get to know them and form a friendship and all that wonderful stuff that comes with being polyamourous
but they have a negative association with Partner B now becasue of their 'breath holding'

they are sad that they arent more exicted for me, but they just arent
they cant will themselves past the broken trust

TL:DR
Im looking for advice on how to rebuild trust around the idea of meeting new people and forming new relationships outside of my Anchor/Nesting relationship