Maybe…
Abuse, definitely the abuse. She doesn’t have Kairi’s memories due to her unusual birth, and I doubt princesses with pure light have insecurities.
Well looking at the New Princess, there's tons of insecurities there.. With Kairi, her insecurities could be that she feels too weak to help and responsible for all that's happened to her friends, obviously she wasn't to blame, but she still might have blamed herself.
Also while Naminé doesn't have Kairi's memories, people in universe have noted that she has similarities in personality, which makes me think there's some influence between the two.
It's the mistreatment. Naminè was literally born into slavery with no knowledge of anything except what she obtained from looking at Sora's memories. She spent her entire year of life tucked away in buildings.
It would have been nice to see how her first days were and how they discovered her.
I imagine she was also in Zombie Mode for the first week or two like Roxas. I'm kind of curious as to how neglected she must have been to have such crippling loneliness after just ba month of existing.
I have no doubt whatsoever that Naminé is a wielder because everything that applies to Roxas being able to wield because of Sora also applies to her, regardless of how much of a Nobody she actually is.
I imagine that had the Organization know Kairi was a wielder before Riku got her the Keyblade, things would've been drastically different, lol.
(which brings the question of how did Riku even know she was a wielder in the first place?)
I'm not so sure about her being able to wield seeing the key difference between her and Roxas is that Sora already could summon his Keyblade while Kairi couldn't when they were created.
Also I don't think, her being a Keyblade wielder would change them trying to use her to manipulate Sora, with her memory powers, though she might have been treated better, seeing she'd be viewed as more valuable. Maybe Larxene and Marluxia would have tried to make her their pawn?
How Riku knew Kairi could wield and summon the keyblade for her is a mystery that I doubt we'll ever get answered.