I have problems with this, too.

When I'm out with a friend, generally the coversation goes back and forth with "How are you" and "What's new". I usually pose the question and let my friend answer. Then I respond as if she's asked the same of me.

Even if it's just short like, "I'm having an off week, but hopefully next week will be better."

You don't have to go and spill everything, but just say something small that's in line with your life.