D.B.
If she believes she is a freak, she needs better counseling, not less.
Forget about grades. Her As dropped to Bs? No big deal unless she herself is all upset about it. Then you have to figure out why she is so upset about that. Fear of failure? Is that why she reacts to being told she did something wrong?
Being an introvert is fine. Being afraid of the world and others is not. Isolating herself in her room can be a telltale sign of depression. When she's at school, are you checking her room to see if there's anything harmful going on?
If she loves to draw and likes coding, perhaps she could pursue ONE of those things in a very small, quiet group. Maybe a local museum has classes? You could let the instructor know that she reacts poorly to criticism right now. Or, perhaps an in-home tutor for one of those things, dedicated just to her. That might help her build confidence.
I think counseling and a life coach (or a combination) are in order. She's 16. She cannot go out into the world like this, and she can't function in her current world like this. I know she doesn't want to talk about it - but that doesn't mean she shouldn't. Maybe she needs a different counselor. And maybe someone designated as a "life coach" would be more palatable to her.
She needs a backbone, and it's up to you to find someone to help her develop that. By the way, staying in a bad marriage isn't good for kids. They know there's something wrong, whether you've told them or not. It can work the opposite way of what you intend, showing them that people should wallow in their misery - which is exactly what she is doing. It shows that people aren't willing to do something difficult to make their lives better - which is exactly what she is doing. It can burden them by making them feel responsible for their parents' unhappiness. They may misread their parents' frustrations and angers at being directed at the kid, rather than the other parent. Think that over and get some serious counseling for yourself and your husband about this.