I can feel your sadness through your writing 1Mom. As a formerly divorced parent, I know how hard it can be. I would advise you to shift the focus from working on the relationship to working on yourself. Go for individual counseling, do some soul searching, take some me time. The answer to your question "how do you determine that enough is enough" will become much clearer once you have done some self examination and worked on yourself. Plus, i think your child will be happier with a Happy Parent--single or not.
Wishing you all the luck in the world--and much happiness in the future.