If she hits the bottom, will she bounce or splat?

I am a typical depressed, anxiety ridden person. I know that if I do not deal with these, at least to a certain degree, I become a toxic person—to myself and others. I’ve also been subject, dare I say victim, to others toxicity.

From these experiences, I say to you to rid your life of this person. Don’t expect it to be emotionally easy, as they will fight it. And they will guilt you, rant at you, blame you. However, the relief you feel at the onset, will prove you are doing the correct thing —for YOU. And remember, it’s YOUR life that matters.

Almost everyone deserves a second chance (a third sometimes), so tell her the truth of your feelings (if you need to, do so in a letter/message/text) one last time. Tell her the parameters in which you need to keep the relationship. Give succinct guidelines of what you will permit, or not.

Although, in my experience, they won’t change for you.

Try to also remember that you CAN NOT help someone who does NOT want to help themselves. It’s an uphill battle, that can never be won.

I hope any of this helps you.

As someone currently dealing with depression as well as someone who has suffered on tube opposite side of codependent relationships, I get your frustration. My best advice to you is to state to her simply and clearly the next time she reaches out that you are not capable of offering the support she needs and suggest, one last time, that she seek out a professional who is trained to offer the help she needs.
You might also mention that you cannot put her mental health needs ahead of your own and that her current approach has become emotionally toxic to you. If you are aware of specific resources you may even offer to accompany her to an appointment with the understanding that she must continue counseling or lose contact with you completely.
None of this will work unless she wants it to, and be prepared for backlash, because it's far easier for her to call you a bad friend for abandoning her than it will be for her to admit her own part in the problem. That said, don't attempt to direct blame at her or become defensive because that will only reinforce her erroneous perspective.

Remember that you can only control your actions, not your friend's. If you have to reduce your contact with her for your own wellbeing thats what you have to do. You've done what you can for her, but its her life to live

That's such a difficult situation: I've been in it many times, both as the friend of the depressive and as the depressive. I think being the depressive taught me most: at my worst, I was so negative that whenever a friend tried to help me I'd basically spit their well-meaning words right back in their face. When I got a bit better, I felt absolutely mortified about some of the things I'd said in reply to well-meaning people who cared about me, and quite frankly I'm amazed none of them refused to have anything to do with me ever again. And very, very thankful of course, because even though I was unable to appreciate what they were doing for me at that time, those friends pulled me through several deeply shit years and I wouldn't be here right now had it not been for them.

If you offer to help her, she's likely to reject you - but it really is the depression talking, not her. Now that I'm over the bad years, I find I'm better able to cope with friends who are going through what I went through; when they make me want to punch things and scream "YOU FUCKING UNGRATEFUL BITCH!" I tell them I'm there for them, and I know that there's a part of them that truly values that - and then I go home and punch the wall while screaming "YOU FUCKING UNGRATEFUL BITCH!"

One day - I hope - your friend will be able to say "thanks for being there."