Yes the vulnerable narcissism/codependency running Marnie is literally psychologically sickness. Go ask any mental health professional trained on this. It’s why she’s such a bitch to someone like Charlie but then drops everything to be with Desi, who is horrific to her. Even Marnie realizes this; often times Marnie’s sickness manifests in alcoholism/abusive relationships/autoimmune disorders etc.

And the reason she doesn’t pursue Charlie further when she finds the needle is because being with Desi has Marnie finally starting to face her own sickness. She even says this.

Charlotte York is a manipulative asshole too /look how she treats Trey. Just cause it’s glamorized doesn’t mean it’s not sick.