Smothered Only Child

Gunborg,
I am a 22 year old, only child who has been living at home with overprotective parents forever. I am currently dating a girl with two kids and I will be moving into a new apartment with them in less than two weeks.
My problem is this: My parents have no idea that i am even dating this girl. They know who she is vaguely, but from their reaction to knowing that I even associate with a girl who has kids, I was afraid to tell them that she is my girlfreind because I knew it would have been a never-ending fight. The problem is that they have a very specific picture in their minds as to what my life should be like and so anything I do that they see as straying from this image, they refuse to accept. I chose to move in with my girlfreind and her two kids because A) I need to finally move out and, B) I want to force my parents to look at me as an adult who they cannot control.
Now, I can already see how this conversation is going to go, and it is going to include a lot of crying and "YOU DON'T LOVE US" coming from their end.

So my question to you is: WHat do you think is the best possible way I can break all this news to them?

-Smothered Only Child

Write a letter where you tell them that you want them to accept you the way you are or stay away from you.