How could leaving an unhealthy unhappy situation be even more unhealthy for a person?

You sound scared to be alone, which is very common. But, clinging to a relationship, an unhealthy one at that, out of fear of being alone is no reason to stay.

I am single and am happy on my own.

Yes, it took a year after my divorce finalized to get to this place mentally and emotionally, but I am far happier than I was being in an unhealthy and unstable marriage.

Being alone and single has many advantages. You don't have to answer to anyone but yourself. You get to live your life on your terms and no one else's. You get to do what you want and when you want. And you get to build the life you want, without any interference or preferences coming from someone else. It's completely up to you, how you choose to live your life.

So, get your courage up and rip off that bandaid. Staying in an unhealthy relationship can be far worse than being alone. We have only one life to live. Would you rather throw in the towel now and give up all hope of being happy? Or would your rather be brave and have a chance at happiness again in your life?

It's all your choice.