Crossing his heart is definitely making promises to others and crossing his wrists is most likely cutting himself. He just wants things to change.
I believe Miya is another friend of his. Either she's suffering similarly to him or she has already suffered and he is self-conscious around her because she's already gone through a lot and she is sighing because he's being over-dramatic and she thinks he should just get over himself (or he thinks she thinks that).
Cathartic to the max
Crossing his heart is definitely making promises to others and crossing his wrists is most likely cutting himself. He just wants things to change. I believe Miya is another friend of his. Either she's suffering similarly to him or she has already suffered and he is self-conscious around her because she's already gone through a lot and she is sighing because he's being over-dramatic and she thinks he should just get over himself (or he thinks she thinks that). Cathartic to the max