Ah! The guy I'm talking about having a primary relationship with also suffers with depression. I think that condition untreated just makes things really hard.
You may have something with the "primary relationship taking all my stick-to-it-iveness" theory, but it seems just as likely to me that you just have a better fit with your husband than with the new guys you've met so far. Plus, it's always going to give you much more pause to let go of an LTR versus something that's very new. I think that'd be true even if you were newly single, mono, and dating.
There's nothing wrong with being choosy and not investing in something that's not feeling right.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.