The story concludes not with a "cure" for her sadness, but with the hope that she can now face the daylight without fear. If you'd like more specifics, I can help with: for the leads A breakdown of the visual style and animation Where this fits within the drama/josei genre
It looks like you are referring to ( Sunflowers Bloom at Night ), likely the OVA (Original Video Animation) connected to the Himawari! anime series. Himawari Wa Yoru Ni Saku -OVA Sunflower Ha Yoru...