You can probably only answer that with any real insight until you've lived one. I haven't yet. That is, I haven't completed this one yet and I'm eighty-one. Perhaps a more relevant question is, "How can I live a life where I don't feel the need to ask such questions." That is, I don't feel so down that I find life arduous rather than joyful.
In answer to those questions, that is, How can I be happy much, if not all of the time, a great deal has been written by people who have found out how. So maybe you could start a 'meaning of my life' search right now. You only need to seriously determine to make a start.
You need to remember that the question you ask is a common one. You have no monopoly on discontent, or deep unhappiness. Just about everyone goes through it. Sometimes more than once. Start thinking about what you can do to use your own particular skills, gifts, qualities to help others who might feel the way you do now, and the problem will solve itself.