Leta never stopped blaming herself for letting Newt take the fall for the Jarvey incident and abandoning her dear friend afterwards. But it was either that or Newt seriously getting hurt by her sadistic husband-to-be and her family.

Her chance to finally redeem herself in his eyes comes with Grindelwald's rising power in Europe. Newt gets himself in a spot of serious trouble. His brother and friends are searching for him. Leta might not have been strong before but with Newt's life on the line she stands up for herself and helps his friends rescue him by going against her family.

+ 1 Newt never blamed her. He might have been infinitely sad (which Queenie picked up on in the movie) but he never once was blamed her.
+ 2 Leta telling Dumbledore what really happened back then even if it is 15 years late.