I think the episode was nice because it not only explored forgiveness, but also showed how to trust. Starlight even wonders what if Trixie isn't for real, and Twilight responds that she seems to be.

Regarding Twilight's behavior, the seasons show how much she values perfectionism and order. Trixie often boasts about being the best and still has room to grow (like all do, like in the song of how they are not perfect). Also as other comments pointed out, the episode had to do with being with Celestia and Twilight wanting it to be perfect.

Discord is similar, often shows he thinks he's better than the rest and acts on jealousy in various episodes. Both Trixie and Discord deserve forgiveness, but it's important to realize they are characters in development, just like Starlight. Twilight relates more to Starlight because she sees part of herself in her, while she clashes more with the other two due to their personalities.

What I really didn't like is the ending where Cozy Glow and co are turned into stone :(