I had some terrible anxiety after I quit too, health anxiety specifically. I found that I actually needed help with that too as anxiety is a potential health issue itself.
Regarding the "what if's" though....as Dee says we can't change what's already happened. You've made a great decision to quit and it will certainly benefit your health and many other areas of your life. Focus on that if you can and move forward....talking to a counselor would be a great idea too.