I'm an overthinker too
It's getting messy isn't it? Any relationship is only as good as the honesty that holds it up, and that's even more true of Polyamorous relationships. How can you release boundaries if you're both lying to each other?
If you want this relationship to work moving forward it will have to be squeaky clean. I don't believe there is anything wrong with checking up on partners from time to time. I use my partner's ipad for research and will sometimes scroll through his emails. I told him I do it and he was initially a bit surprised but if he has nothing to hide, he has nothing to hide.I have nothing to hide and it feels very liberating after a very dishonest marriage. If he asked me not to for any particular reason we would discuss it and if his reasons were valid I would respect his wishes. I have only arrived at this point however, because he has proved himself very, very trust worthy. Trust doesn't just happen it is earned, especially after previous dishonesty.
These things that come up are opportunities for your relationship to grow, but they are more than that they are the things that will make or break your relationship in the end.