Erattic (adjective) – characterized by lack of consistency, regularity, or uniformity

Source: https://www.merriam-webster.com/dictionary/erratic

From: “The Cheater’s Guide to Love” by Junot Diaz

“You’re fine for, like, a week. Then your moods become erratic. One minute you have to stop yourself from jumping in the car and driving to see her and the next you’re calling a sucia and saying, You’re the one I always wanted. You start losing your temper with friends, with students, with colleagues. You cry every time you hear Monchy y Alexandra, her favorite.”

The author uses the word erratic here to show how the feelings of the narrator are conflicted. He is unsure of what to do, and when to do it. The narrator’s fiancée leaves him after she finds out that he is cheating on her. Now, he is going through a period of grief and he wants to try and get her back, but a part of him wants to be with other women.  So, he is unable to maintain a consistent mood.