As someone who (unfortunately) relates A LOT with Adachi it's kinda sad to see people calling her insane but maybe I'm overreacting, idk.
I'll preface this with saying that Adachi's behavior is excessive and very overbearing but whilst it may come of as "insane" her actions are based on logic, twisted as that logic may be.
She is a victim of neglect and thus never got taught how to actually manage her emotions and was never really taught that she has value and thus naturally she doesn't value herself.
Her emotional age is way younger than her physical age, most of her reactions are things you'd expect from a child.
Speaking from firsthand experience, it doesn't matter if you spend large amounts of time alone trying to "fix" your emotions, without actual experience with people who treat you well it's VERY unlikely that you will actually start valuing yourself and treat yourself with respect (at least if your issues stem from the same source as Adachi and you exhibit a lot of the same behaviors". This lack of self respect helps explain why someone like Adachi would cling for life to the things they love, it's like going without water in the heat, whenever you find water you are gonna drink more than you need too since you don't know if you'll actually ever find more and then the cycle repeats. Just in Adachi's case it's loneliness and then extreme reactions at the first sign of emotional investment and love. You desperately need experience to grow which is hard to achieve the more unstable you are.
Shimamura needed intrigue and a call to action, she was always capable of dealing with her problems she just needed/needs to learn what they are and figure out what she values and who she wants to be.
Adachi needs are... more complicated. She needs external things like love, patience, structure (as to not let her get her way every time she gets emotional whilst the other person also has patience and understands that recovery and self improvement is a journey) and time.
Her emotional development is so nonexistent that she needs other people to help lay the groundwork so that she can truly start standing on her own legs. That's not to say she doesn't have a responsibility for her own behaviors and reactions, she 100% does, but the things she can actually work on by herself are things that most people don't even
see as possible issues.
Adachi does try very hard to cut back on how clingy she is when she can actually conceptualize it. Her base emotional state is really intense which makes her act out at inappropriate times for things that might seem (and are) small but in her mind is huge, kinda like a child.
If Shimamura gets the importance of structure and standing to her beliefs and Adachi gets more self insight and learns to value herself they are actually kind of perfect for each other.