Hello jillybeans, it's very telling that all your reasons for staying are due to fear of being by yourself and not to do with the marriage. Of course it's going to feel overwhelming after 23 years, and after one attempt of 3 months that didn't work, it makes sense you're feeling stuck. But perhaps the 3-month stint can be something to learn from. Why don't you think it worked? What could you do differently next time to help you ride the bumps (because it won't all be plain sailing)? I left a husband after nearly 20 years and I can see myself in what you're saying, my fears were also around, how will I cope rather than what will I be leaving behind.
My solution was to stay with friends for a while until I transitioned into moving into my own space. That way, I had a built-in support mechanism from which to rebuild my life.