One of the challenges that you will have to face is accepting that there is going to be pain generated by this separation. Either he is so full of anxiety that he is unable to even deal with the thought of separation, or he is taking advantage, knowing that he can defy you without consequences. Or maybe both. It may be difficult for you to know what is going on without help from a professional. Definitely seek help for yourself (don't count on getting him in at first) and your counselor should help you take the steps that you need to start moving toward a healthy separation from your adult child. It may be that will need to be on anti-anxiety treatments before he is strong enough to stand on his own feet, but it doesn't sound like he will be able to move in that direction without strong motivation from you.
But either way, if he is so openly defiant as your account suggests, it sounds like you may have had trouble in the past in delivering on promised consequences. (Forgive me if I am making bad guesses, it is only a guess and I do not mean this to sound judgmental) Punishment is a word that leaves a bad taste in many parents' minds, as it brings to mind abuse and harm, but properly applied discipline is essential for creating a healthy relationship with a child.
Society has consequences. Children who were not consistently punished for misbehavior can grow up not understanding (or not believing) that there will be consequences when they break the rules.
David Boshton's suggestions about laying out the rules is a good one. To this I will add that you need to set consequences for his not following the rules, and MOST important, you need to prepare yourselves to enforce those consequences. It will be difficult, even painful, for everyone, but it must be done if he is to achieve the independence that he needs.
What are you prepared to do if he refuses to pay rent? You may have to go so far as change the locks and not give him a key. That means if you have left the house and he gets back from work he will have to sit outside in his car waiting for you to get home. He needs to be made to understand that the place you live can no longer be his home, it is your home and he is a house guest. This will be easier once you move and he no longer has memories and a sense of entitlement attached to the house.
We all love our children, and it is painful (sometimes even more for us than for them) when we must make them unhappy. We take away something they want and believe they deserve, and they cry or they rage. I will always remember the look on my best friend's face the first time her child threw a full-on tantrum, screaming "I hate you, I hate you, you're MEAN!" and ran into her room, slamming the door. It stabs us through the heart for so many reasons. But every parent has to go through it.
Most vital of all is understanding what expectations and consequences are appropriate, hence the counseling. Personally, I think a couple of months of counseling should be mandatory for every new parent, and that occasional refreshers are healthy and beneficial. Not only will counseling help you to find the right path in this relationship, it will give you the peace of mind to know that you are doing the right thing.