Before you became official with your friend, the stress wasn't there. Now that you are with him, you are trying to appeal to what you have known him to like. Changing how you are, even if physically, to appeal to your own fears of not being good enough for him, seems, to be playing on your mind. He did like you before this. Which, I know, isn't what you wanted to hear, you clearly stated that.

And now, you are considering ending it, because, it's?? too stressful to be something that you aren't? And for fear of getting hurt, hurt him first?

I am not sure, after hurting him, your friendship will go back to being the same? Maybe, he wanted a friend first, girlfriend second scenario? I am not sure. How did he come to decide involvement with his best friend? Isn't that the ideal of a relationship? Being best friends?