The failing predates WoW though. And I feel like that may be part of the point.
Arguably being told she keeps failing over, and over, and over again isn’t just encapsulating the events of WoW, let alone just her plans in BfA. It thematically hearkens not only to Lordaeron and Teldrassil, or failing to enslave Eyir, or failing to kill Arthas herself, but all the way back to the root of her own loss and despair, when she led a doomed fighting retreat against the Scourge. As sad as it is, while she may have managed to infuriate Arthas with the delays she and her Rangers caused, ultimately there was no hope of victory for her. Events outside her control - namely the betrayal of Dar’khan and resultant collapse of Silvermoon’s magical defenses - had already doomed the high elves, and so she was never actually in a position to legitimately end the advance of the undead and prevent the fall of Quel’thalas or her own death. So she was forced to fight and retreat, fight and retreat, in a grinding spiral toward the inevitable end once there was nowhere left to retreat to. Failure, after failure, after failure, without hope of success.
Hence her repeatedly flipping out whenever she’s told she can’t kill hope. It just doesn’t compute for Sylvanas. The very idea of hope is anathema to her, because for Sylvanas, hope was killed in the forests of Quel’thalas the very day she died and came back as a puppet of the Scourge, and there was nothing she could do about it.
Which is likely why she’s so hung up on denying hope, while other Forsaken still cling to it. Sylvanas was the Ranger-General of Silvermoon. She was supposed to save herself and her people. It was her entire identity and purpose. And when that was supposed to matter most, it didn’t. Her skill and experience and position meant nothing. Through no fault of her own, she failed, and failed hard, and countless people - including herself - suffered, died, and then continued suffering because of it. Most of the Scourge’s victims were just regular people who had no expectation of being able to prevent themselves from suffering such a fate, but Sylvanas’ failure, death and subsequent torment was a rebuke of everything she thought she was supposed to be.
It doesn’t make her good, or after what she’s done since even sympathetic at this point, but it does provide a rationale for how she could see the world so differently from even the other Forsaken, despising the very idea of hope while so many of them still desperately hang onto it. Since becoming free they grab onto every tiny improvement as grounds for hoping to improve even more, while she sees every failure along the way as further reaffirmation of her own conviction that there’s really no hope for her or anyone else.
It’s a warped sort of irony, because she believes there’s no hope for anyone, when until only just now she was the source of hope in the minds and hearts of her own Forsaken.