All is forgiven. And thanks for intervening Vixen, this couldv'e gotten bad

Well to answer your question, the only way I can word it is that Simba's childhood doesn't seem enough to make him that over protective. I know his dad died, and that he blamed himself and all, but it's a strange way for all that insecurity to come out. I think that if he was, say, super keen on teaching Kiara and preparing her to be queen, thus preparing her for the time that he is not there (not saying that he didn't, but his main trait was over protectiveness, we must agree) more than sheltering her and kinda making it harder for her, that would have worked more.
To explain this deeper, I must break it down into small bits.
Mufasa dies - Simba feels responsible, doesn't feel good enough - comes back, has Kiara - becomes overly protective of Kiara, somewhat stunting her.
to me that doesn't make sense.
BUT,
Mufasa dies - Simba feels responsible, doesn't feel good enough - comes back, has Kiara - teaches her well so that she can be an awesome Queen without him.
This would make sense.
AND,
Mufasa dies - Simba feels responsible, doesn't feel good enough - comes back, has Kopa - Kopa is killed by Zira - Simba becomes overly protective of Kiara.
To me, this makes sense too.
I just personally think that Simba's hardships and Simba's attitude towards Kiara don't exactly match up. And the theory of Kopa makes sense - losing a child would probably make you over protective of another.
That is all.
But still, Kopa and Mheetu don't exist in the canon world.