That outro/the ending credits certainly have an undertone of, well, loss, I would say. I don't know if it references anything in the original series or the OVA, but by the imagery in the scene there are a few things that work. Loss of herself or her identity, loss of her alternate identity after growing up, loss of her family, or a more general loss of youth and childhood or innocence (like in the Minako-centric episodes of Sailor Moon where she wonders if being a Sailor Guardian is worth losing your hopes, dreams and teenage years over).
It seems like it would make sense in a series that is basically about a really young girl having the power to transform into a teenage magician who is the star of her family's magic outfit.
Perhaps she's lost the ability to transform in the ending scene where she's just sitting in her room staring - presumably vapidly - into nothing. A thousand yard stare after the loss of her ability (that she suddenly seems to morph back into her red-haired self while looking the same as her teenage magical variant seems to hint at that, at least) ruined her family, a steep fall from fame and fortune. With their family's savings gone and no future, her father commited suicide, leading to her mother having a broken heart and mind, and losing the brother to addiction and crime. She's blaming herself, wondering where it went wrong, and pining for happier times.