You probably can't let go because you're not ready yet. Be gentle to yourself.
Being codependent is like being an addict, except you're addicted to a person, to what you get from being with them...a sense of importance, validation, perhaps even of superiority.
If someone is trying to quit anything cold turkey, be it cigarettes, alcohol or in your case, a toxic relationship (you know the one that felt soooo good once upon a time but now feel soooo bad and yet you can't stop going back for more punishment???), you would tell them to get some SUPPORT. Quitting anything cold turkey is hard. You're breaking old habits and creating new ones.
Since there's no "Codie Patch" that I know of, Al-Anon, SR and counselling are your best bets.
It's ok to go through the stages of grief as you mourn the end of your relationship and come to realize that it was not at all what you thought it was. Give yourself a break.
And keep posting
