Oof this hit right in the feels. I had a somewhat similar experience. Sometimes it feels like you're unable to change for your partner, and you feel guilty because they do their best for you and your relationship.
I mean it's quite realistic. Both of them never moved on from each other. Black-haired girl must've been confused for a while that's why she went to some guy, found out it wasn't for her and realized she loves Megane.
I wouldn't really call it completely toxic.