This is fascinating. You've shared some of your fixed ideas about yourself with us. You see yourself as Intelligent and Practical. You and your friends feel that as you are aging, you are becoming more fixed. More rigid.
I think there is some truth in this physiologically. Our brains reinforce the connections we use most often and those we don't wither. So adults can be less open and changeable than children and teenagers. Philip Pullman explores this difference between innocence and experience in 'His Dark Materials'. In Lyra's universe, souls are visible and take animal form as daemons. Daemons of adults take a fixed shape and those of children change shape. When Lyra speaks to a sailor about that he says that its comforting to him to know that his daemon is a gull, because he's a tough old thing that can always find a bit of food and company. The self knowledge is comforting. But he says that others are dissatisfied with their daemons too (usually because they wanted something more impressive than they have). I think there is something comforting about developing that sense of self.
I've defintely grown in confidence. I'm now 30 and feel more sure of myself than I used to. I think experience can do that.
But I've also changed a lot in my habits which can become part of my identity too. For the previous two decades I dodged exercise whenever possible. I'm obese. I didn't feel like I was the same species as people who expended extra energy for fun. And yet I've been running for 7 weeks and I'm loving it and I'm glad that my sense of self wasn't so rigid that I was unwilling to try doing something new. I feel like I am a runner now and my body is beginning to adapt to this new activity and identity.
When I got pregnant with my DS (now 4) we had spent so many years trying unsuccessfully, that it took a long time for me to really feel like I was going to have a baby, and that I was really going to be allowed to keep him. Part of my identity had been bundled up in 'I'm never going to have children' that it took quite a while of having one for me to rewrite it.
Identity definitely can be reshaped, either consciously, like when I decided I was a runner after my first run and made the decision to run three times a week. Or by circumstances which happen (death, disability, redundancy etc).
My husband has quite rigid beliefs about himself, some quite negative and I think it is related to his depression. Where as I can really see the positives in being open, fluid and accepting. I don't yet know how the rest of my story (or those of the people around me) goes, so I try not to have a fixed vision of how things 'should' be. I think this isn't resignation to fate, I think it gives me the freedom to learn and grow and become my best self.
So I think there is some wisdom in both ways of thinking and the lived truth is probably balanced in the space between that rigid unchanging persona and the completely fluid, everything changes idea. Having a comfortable idea of who you are is comforting, as long as you don't let it limit you, or make you feel completely lost when you go through a life change outside your control.
Maybe your reaction to what she said suggests that whilst you are comfortable with your feeling of grown up self at the moment, maybe you are curious about some of those paths not taken yet. Whether that's learning about philosophy or trying yoga, or finding time to read more books. And maybe you are a bit anxious at the idea that you are vulnerable to change too. It is an unsettling thought but every person is in the same boat.
I'm an artist and have a lot of bookshelves too. 