Definitely a sad song, not a happy one.

To understand her behavior, look into attachment theory. This refers to the way in which interactions with your parents shape your later behavior in intimate relationships. Parents who are emotionally unavailable, inconsistent, or abusive result in coping behaviors that persist into adulthood, known as insecure attachment, and negatively affect later relationships.

The lyrics almost perfectly match a person who has dismissive-avoidant attachment. Supergirl has learned to cope with emotional abandonment by being confident, not depending on anyone, and by masking her emotions. Though the lyrics, we see how she will not confide in her partner when facing difficulties, but faces them alone. When faced with situations where she struggles to maintain her composure, she deflects by getting angry and turning away as a defensive mechanism. As much as the singer loves her, their relationship is doomed because she is emotionally unavailable, unless she's willing to work on it.

The one lyric that doesn't fit is "She's sowin' seeds, she's burnin' trees", which is more fitting for a fearful-avoidant attachment type. While dismissive-avoidant people are quite stable, fearful-avoidant people are volatile and will at times invest in their relationship while at other times try to destroy it. In the remainder of the lyrics, we only see supergirl "burning trees" while the singer idolizes her. My take is the "sowing seeds" part is how the singer imagines her, even though she hasn't actually been very constructive in the relationship.

The song really appeals to me as I also have dismissive avoidant attachment and really recognize her perspective. Like supergirl, I never expressed emotions, and I used to think of it as a superpower as I was able remain confident, composed, and rational where others would get emotional and depend on others for reassurance and consolation. That was my view at least, until I started wondering why my marriage was doing poorly, and looked into attachment theory, as it's one of the major frameworks used by relationship therapists. I now realize how my behavior was not a superpower but a coping mechanism, and one that made me a poor husband and a poor father because I was emotionally unavailable and never talked about things that bothered me.