There isn't really a wrong side here. Dark haired girl wanted to be what she thought would make her a better person/girlfriend. She thought she was doing what she could to be better for their future.
From light haired girl's perspective, she saw a lot of guilt and watched her change. Based on my own experience, possibly even in an unhealthy way. She simply couldn't adapt to her lover.
If you ask me, light haired girl should have voiced her concerns, and dark haired girl should have trusted in her lovers feelings for her.