In order to solve any problem, I think it is necessary to figure out the cause. And this also is the place to start in this situation. I am quite confident that you haven't always felt this way. So, the question you have to ask and answer is quite simple. What trail of events led to this feeling I am experiencing presently? This might take some time to answer. But once it is done, you can then do some steps retracing and decide what efforts you must make to salvage the error. They also say that all a man needs to be happy is someone to love, something to do and something to hope for. So, if any of these three is missing, you can also consider fixing that.