I've been learning about Borderline Personality Disorder (BPD) and Complex Post-Traumatic Stress Disorder (cPTSD) and where it may come from. A lot of childhood abuse may develop as BPD and/or cPTSD.

A couple of things that can manifest as a result is suicidal tendencies (Calum says he didn't expect to even make it to 20 let alone 30), impulsivity and recklessness (his arm cast, walking in front of bus) but as well as the anxious attachment style that develops which we see with Callum's relation with his ex/Sophie's mum but he obviously loves his daughter. Anger that can be internalised but explode outwardly (the scene where Callum spits at himself in the mirror or when everyone sings Happy Birthday and he looks so.fucking.angry. Mood swings are also manifested too and Callum does switch moods a lot.

Maybe projecting my shit onto the character but I found this a hard watch with this lens.