I was working with a writer earlier recently who spoke about a first person narrative she was writing and how it began to feel like it was trapping her. Her characterisation was developing well but: she was bringing out the character's backstory piece by piece (rather than dumping a block of it in exposition), and she was also getting some sense of the character's conflicts by listening to their dialogue in their relationship scenes with others. (Her lover, her co-workers, her family.) But with all that sound writing practice the character was feeling one-dimensional.
Sometimes a jolt in perspective can help here. As an exercise, switch to a narrative in first person POV from one of the other characters in your protagonist's life. Set up a dialogue scene and give it some attitude and edge. A flashpoint of conflict, a moment of covert conflict. Think of unfinished business (backstory) in your protagonist's life and design a real-time scene to confront them with it, try and bring it out. But focus on the other character's POV. What does your protagonist do with their body (their gestures, their eyes, the tone of their voice.) What do they let slip out in their words? Turn the tables on them. Challenge them, read their responses from the outside. Sometimes our characters in First Person POV enlist the author as part of their natural cloaking mechanism. We need to break beyond that, to achieve a rounded characterisation.
Don't try and achieve this kind of insight with straight telling, it's always a lesser option. Eg. 'And I knew then I'd always resented my mother...' etc. Bring that kind of conflict out in gesture, in dialogue that circles then pincers, in passive aggressive responses. You're not writing an encyclopedia, with an aim to get information across as quickly as possible. You're looking for insight and revelation, for uncovering.
So when constructing character in First Person, consider an exercise where you switch to someone else's perspective to jolt some new understanding from your protagonist. It's better to do it in another first person POV, rather than third person, as in third person your character might still control your perspective so you don't learn anything new. When you've got some critical insights, feed that back into your main narrative, in first person from your protagonist's POV.
The photograph above looks like a shot of mars, jupiter and venus, maybe from somewhere in the red center of Australia, or Arizona, in a dusty sunset. It isn't. The image is taken from Mars and one of those little starry dots is the earth.