I'm not sure Writer's the one with codependency issues. He strikes me more as needy and manipulative. branches, you're the one who's letting your needs be steam rolled just so Writer doesn't have to inconvenience himself by working on issues that he doesn't even admit are problems. He's basically addicted to your relationship, and you're enabling that. You're letting his personality issues control your behaviour to the extent that it's causing you severe depression.
First thing I would do is shut down his bullshit self-diagnosis of codependency. You're not an addict or mentally ill, and he's not enabling you to continue being so. You, on the other hand, are enabling him to keep being insecure and overly dependent on your relationship as an indicator of his self-worth.
Okay - in your shoes I'd be looking for ways to regain my sense of self and work my way out of depression. In particular, ways that wouldn't make the current situation worse.
ok what? Seems like you're projecting something you've experienced onto her situation. Her depression is situational because she's letting Writer steamroll all over her needs. She was fine living on her own, and she was coping well enough living with Writer. She coped by spending time with her close friends, which is about the most reasonable thing I've ever heard in my life.
branches, the solution to your depression is straightforward: tell Writer that you're going to see your
friends whenever you want, you're going to take as much alone time as you need, and that if he wants to be with you, he'd better find a way to get ok with that. It's blunt and callous, but sometimes that's required to protect yourself from needy, manipulative people. Whatever feelings that triggers for him can be dealt with as they come up, but the reality is you have severe feelings of depression
right now, and that needs to be dealt with first. He doesn't have to think his issues are a problem, he doesn't have to deal with them at all, but his avoidance doesn't give him the right to drag you down with him.
When you brought up polyamory, he turns to Google, but does he search anything about polyamory or how work on his jealousy? No, he goes and finds a list of symptoms, starts self-diagnosing and bringing up all these issues that apparently didn't bother him before, but are suddenly and conveniently preventing him from dealing with your revelation.
Writer is the problem, like 100%. He's not only holding you back from future happiness, but he's actually dragging you down from happiness you previously enjoyed.
The fact is, these aren't problems that "Writer and I" can work on. They're his problems. Only he can work on them, but first he has to acknowledge they're a problem, second he has to want to fix them, third he has to get professional support to do so. If he's going to deal with them, you'll have to force his hand, and you can only do that by putting him in the exact situation he's so uncomfortable with.
You can love him and support him, but you can't do his work for him. And the more you acquiesce to his needy behaviour, the more he's going to expect it.
I'm all for loving and supporting people through difficult situations, but you have to take care of yourself first. No one else is going to. In fact, to be any good to him, you have to be in a good place yourself. You, unlike Writer, at least know what things you can do to make yourself happy: spend time with your close friends. If he doesn't like it, tough. Nobody but nobody has the right to stop their partners from seeing their friends.