I would answer the first question with a resounding 'yes', in this sense: that overthinking isn't a problem because of the 'thinking' but because of the 'over'. The problem with overthinking is that it takes a good thing, namely thinking, and stretches it out of its natural role in our lives. Thought, when out of proportion with our life, is especially harmful. As one of my idols, G.K. Chesterton, once wrote:
To accept everything is an exercise, to understand everything a strain. The poet only desires exaltation and expansion, a world to stretch himself in. The poet only asks to get his head into the heavens. It is the logician who seeks to get the heavens into his head. And it is his head that splits (Orthodoxy).
Often times people who have a personality that compels them to think, however noble the act of thought might be, shall find in odd moments that they are in a desperate way. I had a bit of an epiphany a couple of months ago that there is a right and wrong way of being right. It is not so much the possession of truth that matters; it is the way that you go about carrying that truth, or if you feel that you have not yet discovered it, the way you go about handling that fact.
All in all, life is about having an adventure. And to do this, you have to come to a point where you can accept that there are some things that you cannot understand. At this point it is important that instead of giving into despair, you come to terms with this fact, and play the game of life anyway.
It is natural to spend our teenage years disillusioned with the structures of society. This attitude often finds its way in even matured individuals. Our culture propagates messages of individualism and rebellion. But I think that when you are at peace with yourself you begin to find that a large part of who we are consists in what humanity is. Instead of looking at your neighbor and judging them to be a part of some dull collective, we should begin to look at our neighbor and marvel that they are quite like us, despite the worlds of difference that we think exists between us. Compassion and love are the truest signs of a healthy person.
I would suggest to you that instead of focusing on whether your thinking is harmful or not (which is a pursuit that, ironically enough, is still dictated by an obsession with thought), take a walk through some parks and speak with people you would not normally speak to. In short, have an adventure.