It's interesting how common this trope is, but the consequences are so wildly different depending on the setting. I just reread 'The Traitor Baru Cormorant', and the way Baru's hidden identity as a colonial agent bleeds into every single interaction—with her allies, her lovers, even the people she's trying to save—is the whole point. It's not just about creating external drama; the secret becomes the architecture of her entire personality. Every conversation is a calculation, every moment of intimacy is undercut by what she can't say.
I've seen it done badly too, where the past is just a bomb waiting to go off in the third act for a cheap shock. The more believable stories show the slow corrosion. The character might become hyper-vigilant, over-compensating in their loyalty to prove something, or they'll deflect with humor that has a bitter edge. It builds walls not just between them and others, but inside themselves. They can never be fully present, because part of them is always back in that hidden room, guarding the door. That internal split is what really sells the relationship strain for me, more than any big confession scene.