I can relate to this. I'm new here, but I'll give my experience if it will help to give you hope:
I was in a very destructive relationship for a few years. I knew it was bad for me. The other person enabled my alcohol abuse (also participated) and cheated on me a few times. I still COULD NOT leave. We would get into arguments constantly. We were on eachother all the time, arguing, disagreeing... I could not stand to be alone.
When I would get frustrated and declare that I was leaving, I would back off, panic, and practically beg for her to stay with me. The very thought of it was terrifying. Ugh, my separation anxieties would escalate and make it even WORSE!
I couldn't help but blame myself. I kept thinking I was the one who screwed it up, maybe I was. But either way, the love was no longer there, not really. My NEED was there...that was it.

Well, she had to force the breakup. I just didn't argue anymore. I didn't beg. I just left. It was one of the better I'd done for myself. It was hard at first, I sunk into depression, but had enough support to get through it. I had found out that being alone was NOT the worst thing that could happen to me. Being stuck in a cyclically unhealthy relationship was hurting me and the other person. I had to really search within myself and wish for her happiness, her true authentic happiness. Realize she really would be happier without me, and I without her. This took A LOT of work.

I don't regret it, it is very sad for me that it had to come to that totally sick place for it to end...BUT...I survived. I was okay without her. I learned that this was a good thing. I learned that I did not lose myself. It was scary as hell, it wasn't easy...but I am here to tell about it, as are many others.

If you're worried about logistics (where to live, finances, who will take the cat or kids etc.) These things will work themselves out; I believe that what truly matters is your health and happiness.

You've already made the first step, you realize the relationship is not good for either of you. You KNOW it needs to end. This is a vital step in closure and separation. For this you are further along than some and it's wonderful <3. I was grateful for knowing this (in retrospect of course). At the time it seemed like hell.

I wish you the best. I understand completely, entirely...You are NOT alone.