Not being willing to open up emotionally can make you feel more isolated. I've also found that it's difficult to know who your real friends are until you are vulnerable with them. Anyone can exchange pleasantries, but when the fun's over and someone's still with you...that's a true friend. Try to open up more with those you trust - be honest when they ask you questions like "how are you?" Speaking from experience, this helps build friendships, because not only have you demonstrated that you trust your friend, you've also shown them that you are okay with discussing negative feelings. Which means that if they're feeling terrible and hiding it from you, they'll now feel a lot more comfortable being honest about how they're really feeling and opening up to you.
I'm sorry you're feeling bad about that