No, I don't understand this, not completely, but I love it anyway.
The colour, first, is very well done and beautiful. It is predominantly palely neutral; the slightly darker skin tones of 'the little creatures' and 'the main character' providing subtle contrasts. On either side, the painting is injected with bright colour: teal blue fading in on the left, red autumn hair on the right. These lovely hints lift the piece, adding both emotion and brightness to the thoughtful and depressive atmosphere.
At the centre are the four 'creatures', incongruous little things occupying their own space of reality through their strange poses and gravity-defying positions. They are the starting point of this visual journey, but the eye is constantly drawn back to them again and again, every time seeing something new. Even without your description, their individual personalities become evident in the lines of their slight faerie bodies and the mixed expressions of submission, disdain, bemused inquiry, and quiet purpose. They are beautiful in a sometimes classic, sometimes imperfect, sometimes unnerving way, and it is this very quality that makes them so striking - and mesmerising.
The main character is different from the creatures, and she holds her own restrained loveliness in the gentle tilt of her neck, the subtle dark flick of her eyelash, and the elegant poise of her hands beholding the lily. You're quite right - the black of her shirt contrasts starkly with the tone of her complexion and further emphasis her ballerina-like grace. It further creates a very dramatic block of dark colour that draws the eye as equally as the creatures. I love that single, almost invisible strand of wayward hair escaping the severe bun to curl behind her ear. It's so slight as to be almost invisible but it nevertheless adds that sense of 'untidiness' even if it isn't noticed at first glance. You might consider making it a little more evident as it tends to blend in with the similarly reddish tone of her cheek.
Not only your major elements, but your background is equally mesmerizing. At first it seems to be simple white space, but it is in fact full of faint textures and patterns. The hints in each corner of leaves and trees, seemingly at odds with the indoor setting behind the girl, adds to a feeling of surreality. But not too much. It is simple enough not to disorientate the viewer with its peculiar content, and revels in that simplicity. the tree branches on the right are especially well-placed, I feel. With such a subtle background image, it would be easy to get their look or placement wrong. They come in delicately, finger-like and not too obviously, gently fading into the pale peach - like sudden fireworks just as suddenly dying out.
I really adore the two origami pieces as well. They act like visual quote marks, beginning with the lily and ending with the fish. It feels like the essentially inanimate lily becomes the fish, travelling through every point of contact and finally emerging as a being of free movement. I'm not particularly sure how this relates to 'Serotonin' particularly, but it's wonderful.
The desk, pen and paper, also done in low-contrasting tones, provide a bit of background information about the main character without distracting from the main images. What does this tell us about her? Maybe that she is a writer, or an artist, or someone simply using this stash to while away the time making origami flowers. Whatever it is, it sets her in the real world. Along with the walls, it ties her to the tangible even as she has links to an intangible reality through the creatures.
Like your other works, Serotonin is full to bursting with details both immense and infinitesimal from the filament hairs of the creatures to the fleur-de-lis like wallpaper. From the origami lily and goldfish to the clawed tree branches reaching from somewhere beyond the edges of the picture. You put yourself into this painting and there is beauty in every line because you make it so. Thank you, for 'Serotonin'.