Something I've noticed with those newer to poly is that there is an assumption that the trust they have built in their relationship with each other over time also translates directly out to any new relationships they are forming. This has proved a bit problematic, as the people in those new budding poly relationships that are changing from previously monogamous to polyamorous are usually oblivious that this expectation is present.
It might not be going to ground zero on the trust factor when moving from mono to poly in your original dyad, but poly is a different dynamic, with different skills needed, different negotiations to have, different agreements to develop, and a different kind of trust to earn.
Yep, I said earn. People can talk about giving trust all they like, but every time I've "given" someone my trust, it's because their words and actions are congruent enough over time that trust is built. The longer this goes on, the more consistent and congruent they are, the more I trust them.
So, you've been in a monogamous marriage/relationship for the past 5-10 years. You've been through thick and thin together. You've seen each other sick, crying, vulnerable, happy, joyous, frustrated, focused. There have been vacations together, children born and raised, pets picked together, bills paid, debts accrued and discharged, mutually impacting decisions made. Why doesn't the trust you've earned there just transfer into your poly life together? Because this is something different enough that it gets its own category of trust-earning, and that trust folder is currently empty, if you're lucky, or filled with things like, "I've met someone that I really think is shiny, and I'd like us to consider polyamory!".
To move from mono to poly, there is a change in existing agreements, which resets the clock after a fashion. The good news is that you *get* to start over. The bad news is that you *have* to start over. If you treat your long-term partnership as though you've got those 5-10 years of historical trust to buoy you up in this new endeavor, it's likely to bite you in the ass. Sure, it doesn't disappear, but this new category needs to be filled with positive shared experience to earn the trust in poly.
How does one accomplish this? Pretty much the same ways you accrued trust within your mono dyad. Do what you say you're going to do. Don't do things you haven't negotiated for. Show up when you say you'll be there. Don't go MIA without notice. Take care of your responsibilities. Ask, rather than assume, if you need help with something. Communicate clearly and consistently without a hidden agenda. Don't make up stories in your head about what's going on with your partner and their new connection(s), instead, ask for information. Ask for what you want/need before it becomes a problem in your relationship.
If your objective is to create an environment that nourishes your vintage relationships, as well as your new poly ones, it's well-worth the investment to build your poly trust bank up. Trust is simple, but not always easy, to earn.