Being in the closet is usually emotionally and psychologically unhealthy. No one should have to feel like someone else's secret. And I imagine it's especially hard for you because they get to be publicly affectionate while you can't. Whose decision was it that openness is not allowed? Any chance that could change down the line?
I second the fact that another option is to find another partner in addition to these two, one you CAN be open about. But tell that person up front about your other partners... in all honesty it was really shitty of your boyfriend to hide things from you at the start.
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.