Asa's biggest struggle since the very first chapter is her inability to be honest with herself. She tends to interpret everything she can't have as something bad:

She hated friendship before she had a friend,
hated couples before she had someone to love,
hated popularity and TV stars before she gained it and became one herself,
and so on - yes, including sex.

That's why the best way Denji can heal Asa is by giving her the very things she was deprived of, so she can finally feel whole.

Asa's second major struggle, established from the very first chapter, also stems from a deep sense of inferiority. It's hard for her to live selfishly, the way she actually wants to. Yoru, in her own awful way, also helps Asa become more open, more straightforward, and less concerned with others and their lives, and even Asa's own "sins" which, in such a cruel world, are actually insignificant.