Y'know, usually mermaids are usually seen as sentient beings. So having them caged up is kinda strange. Why not have her in a grief scene at the aquarium trying to reconcile her self blame and getting the idea to start a movement to free mermaids? To be honest most people in grief just need an outlet and do something other than devote time to their thoughts, and freeing an entire species seems like a pretty big something to me. She could become a woman on a mission.
Maybe she will eventually realise that she's not living the way her parents would want her to, that she has to let go of them for her to move on. This would of course be after several years of counselling and some major turning points in her life that give reasons as to why she shouldn't feel guilty. In my opinion though, this is the usual stereotypical "everything worked out well, wow, how (un)expected" so it's pretty boring.
Another idea would be to involve her in something grand, like an organisation to notices she's a young orphan, and hence practically alone in the world, takes her and moulds her into something robotic. Some soldier/assassin or whatnot. Something that doesn't require emotions. After brainwashing her she could struggle from two different aspects in her personality, the human side that remembers the grief she holds (because that's the most poignant one) and the one newly produced from the training she received. Personally, I don't think this fits very well into your story, it's more of a nature vs nurture kind of thing rather than focusing on a girl dealing with something no child should handle. But hey, I'm not writing the thing.
Other than that, the whole self blame thing isn't something that can be readily cured... Unless you find something or someone to blame it on it won't just go away.
Or, y'know, if you want to take the long scenic route and dive into the deep end you could show her gradually breaking down, falling into depression, punishing herself (self-harm as a way to temporarily alleviate some guilt, maybe through those whip thingies or by exhausting her body over its' limits, pain to forget and the exhaustion because everything blurs and fades away once you're that tired). She'll probably go through a drug/alcohol phase once her body adapts its tolerance to pain and tiredness. It is just another way to escape reality after all. Eventually she'll kill herself because she can't go on anymore. The highs have become lows and she can't stop her emotions from overwhelming her anymore. So she'll decide the only outlet is to kill herself. If she's a religious person maybe she can think that hell would be better suited to punish those like her. Or maybe she just wants to forget. If she's into reincarnation it'd just be like hitting the reset button. No more pain, no more memories. A clean slate so to speak. Maybe she can go via drowning? Or perhaps (if mermaids are carnivores) bleeding herself out and letting them devour her body. Seems symbolic in a way I can't quite grasp but hey, just spitballing here.
So yeah, a few ideas, but nothing really quite notable. Most of it is really just something to distract herself with. Pain fades with time, if it's treated well. If not it'll fester and spread until it consumes. Good luck on your novel.
