Old Wounds is a Very Misunderstood Episode
I think a lot people think that the show was trying to tell us Lin was wrong for not forgiving Su, but it actually was that she was only causing herself pain by continuing to hold onto her grudge, denying herself a relationship with someone she cared about and people that cared about her.
I also think this episode needs to be watched in contrast with The Earth Queen, where Bolin and Mako meet their Dad's family, but find out that they had to grow up on the streets, orphaned, not knowing about a whole family that they could've live with instead if their Dad and Grandfather had just gotten past that their Dad didn't want to work in the family business.
And also, for the people that believe Su never apologized, she tried to reach out for years, Lin refused any sort of contact to reconcile. Su literally apologized to Lin for how she acted as a kid at the end of the episode.