You're trying to make yourself into someone you're not... Basically, you two have different ways of communicating and you don't really speak one another's languages. You might be more straightforward and methodical about how you communicate with him and he might be more sensitive to body language or an unspoken connection. Neither way is WRONG, just different. He doesn't sound like he feels that connection to you when you talk; what he means by you "opening up to him" is you communicating on the same wavelength that he's used to. I don't think anyone could possibly tell you how to be a more open, emotional person, but if that's not your communication style then it just isn't.
If writing that letter was a big move for you and you felt good about it then who is he to tell you it isn't the right way to do it. You need to be with someone who wants you the way you are, not the way he wants to mold you.
Welcome to the board gal_pinklady,
Being able to express yourself emotional is important to some people in a relationship.