Based on my own experience with a boy, the way I changed, and who I was scared of becoming:
Usually, people see the world through their own, personalized, water-proof goggles. Kelsi was under the impression that she had gotten over what happened that day. She had buried the memories deep, moved to a different city, a different state. Until she sees Jason again, and suddenly she is remembering everything. Suddenly, Kelsi begins to realize that she's been carrying the scars with her for too long; The repercussions of that day are evident in too many aspects of her life.
When she meets her Best friends family for the first time she doesn't revise that her biggest crush is standing in front of her. She reviles her passed to him and something shocking happens, and know she might be regretting it all.