I am sorry LoveBunny. When you described your husband as liking routine, not challenging himself, fearing change, and a workaholic I feared the odds of adjusting your marriage to an open or poly one might be slim.
You can't make someone do hard personal work and that is what was required for your marriage to survive. I think your husband realized that he didn't want a real open/poly marriage and that he did not want to do the work needed to be ok with that type of relationship. Obviously his prerogative and probably the best thing in the long term.
That said, it's going to be miserable for a while. Ending a long term relationship, even one that seems to have run its course, is hard and will make you doubt all kinds of things about yourself (that at least has been my experience). The ideas of learning to be alone is a good one and I believe a necessary skill for any adult. And you are now free to chart your own way, to experiment as you see fit. That is exhilarating, if also nerve-racking at times.
Pulling for you!