I think everyone here can understand the feeling of wanting to be happy - of making SOMETHING happen because of being tired of being stuck. I absolutely get that. Shoot, I wrestle with that every day. Every day.
But...one of the best pieces of advice that I ever received was that big decisions should not be made when a person is panicking or in crisis.
You have just come through a difficult conversation with your husband and are waiting for some medical tests. Not two days ago you were in an absolute panic over what to do.
Here are the questions I ask myself when I feel as you do: if I were to fully cut him out of my life, what IN my life would change? I would not be any less hurt, that is for sure. It will not magically go away. I can still do the work on me and have him in my life. We do not have a defined relationship but then do I figure that I am going to hit the dating scene right away? No, not really ready for that.
You can ask your self a few more questions: If your H is in MLC, you have just asked that he help you through your illness and he has agreed. By your own admission, you need him to step up to the plate. Would giving him that goodbye letter be a good idea right now? (I absolutely agree with Snodderly regarding that letter, BTW)
The hardest thing in the world is to just let the relationship play out while you take care of YOUR life. Trust me, I am still working at it.