One of the hardest parts of being a parent is letting them make their own mistakes. I wanted to step in all the time and still do even though my two oldest are married with kids of their own. You just have to come to a point that you give them your best advice as calmly as you can, then step aside and let them fly on their own a little.
I could not be reasoned with when I was young. I had great parenting but could not wait to get out there and make my own mistakes. I made lots of them too, but at least my folks had given me enough to KNOW when I had made a mistake, and I could learn from it, and carry on once again. My oldest was just like me. Could not wait to get on his own. He readily admits making a lot of wrong turns but he has also become a very successful, happy man I am so very proud of! My second son, stayed at home longer, he was less adventurous. I thought he was making some wrong decisions from time to time as well. He too, has become a very happy and successful man who I am also very proud of. I expect my third son will also follow suit and make a lot of decisions I do not agree with. He may be the hardest head of them all

but I have no doubt that he will work his way through and also become happy and successful at whatever he chooses to do.
The point is, you lay a good foundation, let them know how much they are loved and then let them try their wings -- even when you feel they are not making a good decsion. Chance are they will turn out just fine. I let them know the door will always be open.
I hated even sharing my kids with the school systems -- letting them go off to make their own lives is a LOT harder! Hang in there!
Came back to add something. The first time I moved out many many years ago, it was in anger and in with friends. I had just graduated from HS but was not 18, so my mom came and got me and made me come back home. Very embarassing. But she made me a deal, if I wanted to move out, do it calmly, not in anger, after I turned 18 and she would even help me find an apartment and give me some things to start out. She held true to her word and helped me get started in my first little apartment. That was a much better situation all the way around. Maybe soemthing like that would work here. Not the waiting til 18, as he is already 21, but waiting and finding a better place of his own.