Couldn't save her Former Elsa, so she's resolved to never abandon her Current Elsa.
Heartbreaking, but explanatory. She can't change the past, but she can learn from it.
Not a groundbreaking narrative device, but an incredibly effective one for the sort of story beat the author is going for, and appropriate for this story in particular on multiple levels
And hopefully Evie can tie in that, while her past motivates her to help Elsa, Elsa is not a replacement for that girl, and that Evie isn't who she once was, anyway.
Elsa is the most important thing to Evie, and while she wishes to never make that mistake again, it's Elsa herself and Evie's love for her that drive our MC to fight the very world to see their happiness realized.
Thanks for the TL, as always.