Starlight did not reform after trying to travel to the past with Prince Dusk Shine, so she only acts to plan her revenge again., without knowing that she will also fall in love with him.
Twilight Sparkle and Starlight Glimmer are evenly-matched. Every time Twilight tries to stop Starlight's plan, she fails. That means Starlight wins, right?
Current student of the Princess of Friendship, Sunset Shimmer, returns to Equestria for a visit. Twilight Sparkle and her other student, Starlight Glimmer, helps to show her how much better things can get when you learn to let go of the past.
Tired of Sunburst hitting it off with all her friends, Starlight brings in expert of social interaction Sunset to coach her through the process of getting him to sit down and play tons and tons of Dragon Pit. It goes about as well as you’d expect.
When Starlight Glimmer and Trixie decide to stay in the group with the Mane 6, they are able to escape the Storm King invasion and join the Mane 6 and Spike on their Journey Outside Equestria
Twilight Sparkle didn't consider what might happen when she used an "Object Possession" spell to check on her student. Now she has to live with the consequences. Forever.
Despite being redeemed and accepted by the Princess of Friendship, Starlight's guilt for her actions haunt her daily life and it never seems to go away.