I'm revising a YA thriller now, working my way through a lengthy revision from my editor. And you can learn from my mistakes, especially if you are working on a YA or a thriller or even a YA thriller.
1. One problem with a thriller is that it's easy to skip over the sense of the character having a real, full life outside of the problem they are suddenly grappling with. Be sure to give them one.
2. Your characters should be feeling lots of emotions – they are teenagers. Sustain the emotions – they should last longer than just a bit of dialog.
3. Avoid over interpretation if you write in first person. "I realized, " "I felt, " "I could tell that" all should be avoided. Show us how the character feels, not what things mean., ie "I felt like there wasn’t enough air in the room," is much better as "There wasn't enough air in the room."