I feel the same as OP most of the time, I think I just have too many thoughts about how trivial things are rushing around in my head at once, I often wake up all of a sudden with my heart pounding and flailing with the feeling as if I were falling and falling. I find it hard to get to sleep much of the time aswell, I find it very hard to empty my head and just rest.
Even so, in the interest of self preservation and the general atmosphere that surrounds you I find it important to act happy and try and give off an air of confidence. I would hate to think that my mood or inner feelings had brought about similar instances in my friends and family, so I keep them locked. I don't think I'll ever feel any different, and in some ways I don't want to, because I will have stopped thinking and reacting as I do and in a sense cease to be me. Behind whatever I do, there is a deep thought process, I have never just done anything, it is cross examined and picked apart by my brain before anything physical takes place. My grandad recognised this before he died, and told my mum to look after me because I was a deep thinker like himself. It doesn't mean I'm clever/intelligent/always correct but I have thought about my action, and probably a lot of other things beside it.
In my experience, you can't stop unhappy thoughts from crossing your mind but it is how you choose to deal with them and let them affect you that decides how you live your life.