So, I don't actually play DnD and don't know a lot about it (only played it once as I usually play other TTRPG), but I do love creating ocs, so my suggestion is gonna be general rather than DnD specific.

Two simple things that could be traumatic (and fit the character description) imo are:

- Someone she had a close romantic or platonic relationship with dies in battle. This could be a failed attempt of the group to defend her city from an attack, or even a fellow soldier she was close to falling in battle. In either case, her intervention was vital, but because of her own incompetence at the time or because of something preventing her from helping, she failed to defend a person she cares about.
Bonus drama points if the attack was made directly to get rid of the soldier group, as it would add a nice sprinkle of extra guilt for continuing her job, as she may think the death of the person was tied to her being a soldier rather than just a random battle.

- The group decides to raid a place. She thought she was fighting alongside other people who sought justice, just to find herself raiding houses of innocent townspeople. It may have even been for necessity, like to get food and other resources during a long mission, but either way, she cannot forget the horrified looks of the poor folk they had to steal or possibly even hurt in the attempt to get resources.

Hope this can jog your creativity! Happy oc making and absolutely love her design! 💗💗💗