Saying that "the Garrosh you first meet isn't really him" is highly debatable as well know little of his characterization beforehand, save that he was a victim of the Red Pox and seemingly spent his adolescence as a sickly and frail Orcling. Even on recovery from the pox he was described as "tormented by tales of his father," and he "feared he would lead the Mag'har down a dark path." Suffice to say, it seems mopey and whiny Garrosh had a much longer tenure than the fierce warrior we see after Thrall's first visit to Garadar. And to quote Garrosh himself:
I also wouldn't say that Garrosh "was never his own person," simply that the person he was is essentially weak, dependent on the approval of others, and so deep into his own delusions that he fractured the Horde he was supposed to lead, became a tainted abomination, and wound up damned to WoW's version of hell for his troubles. Jaina on the other hand was scarred by trauma, endured a period of volatile instability, but eventually evened out and became a stronger, wiser version of her original self. Garrosh refused to accept culpability for his own actions even in the deepest pit of the Shadowlands, whereas Jaina experienced a more positive growth and actually addressed and overcame her demons. That is what having a strong core of self is.You are an honorable <race>, <name>. You have done much for the Mag'har. No one could ever deny your service to my people. Alas, the time of the Mag'har is at an end. You have shown me, more than anything, that I am unfit to lead these people. My cursed blood runs too deep. I will not... I cannot become the second Hellscream to damn the orcs.
Please, <name>, return to the Greatmother and tell her what I have told you. I am too ashamed to see her... to look into her eyes.