I don't get why it is hard to understand why her character jumps back and forth because the entire lore that is surrounding her character is one big back and forth. She loves Arthas, shes loses Arthas. She loves her father, she betrays her father to forge peace. Theramore gets bombed despite the previous sacrifice she made, she gets mad, she eventually forgives because she tries to stay true to her initial intentions. After she forgives, she gets abandoned by the Horde on the Broken Shore.
Its like when you forgive someone something they do to you, only to see them do something similar to you again. It is obvious you won't want to have anything to do with the person that lost your trust twice.
In conclusion - the character is having proper development, but the plot that she had been subjected to and is still being subjected too forces a back and forth character development. The character development totally fits the plot she's been subjected to. The plot deserves ranting.