The thing is, we've barely been introduced to the characters, we've had a few teenage dramas (the handcuffs, the Holgerson plot) but we really have no idea how Melanie fits in. Darkness comes from someone who wants to stab Victoria's eyes out, who tried to drown her. Vic already has to deal with the idea of being a cripple, but she also has to wonder about Melanie, there is definite foreshadowing there. The sadness of Nia is heartbreaking. Nami's past is dark, Nika has the violent incident to come to light still. Jeff recognises Nika's sad eyes because he's had something bad happen too. Mila's been abused to the point she flinches. Sasha is scarred and takes medication for panic attacks herself. Bella's totally a stable person in all this, who loves being wild and chaotic. Melanie's sister is unhinged and probably happy to ignore the book.
There's plenty of opportunity for drama and failure. But keep aware that even in the darkest stories it's rare to have something end on a complete downer like Empire Strikes Back, or the Unholy Consult. Few people can accept a true failure for the main character. Pure existentialism doesn't sell well. Just as you wish for the victory to be meaningful in contrast with the risk of defeat, the risk of defeat is only meaningful as an obstruction to success. Where there is no success, being grim and doomy is just as terrible writing.
But even then, this is just one kind of story. There are plenty of ways to establish drama and weighty consequence without risk of failure. Oedipus Rex is a great example of that. The inward journey of self discovery, facing ourselves in the mirror is also perilous, because we risk ourselves against the security of apathy. This is primarily the story that Nika faces. Will be deal honestly enough to become a better person, face his flaws that led to Summer being gone? Or will he repeat the problems, or never leave that safe nest of apathy?
Nika acting as a manwhore could turn off some of the LI's. They might not want much to do with him, and might be content not having a romance. People can reject someone and stay single - the love interests don't need to have partners, they could just gravitate to flings and experiment. Nika might fail to establish a sustainable romance, and they rip his heart out callously.
There's so much potential for drama, all that is required is to avoid the cliche's