Learn to generate your own happiness. At some point in our life we need to accept the reality that we can't force someone to stay in our life. We'll meet some people which we will spend our time enjoying with them about a lot of stuff which we may think that they're forever with us. But some leave and we ended up questioning our self "what wrong with us?", "Am I not enough?". So in the end of the day we ended up feeling like alone and lonely. 

You must learn how to provide and fulfill those gaps by yourself in that way it helps you grow as an individual. And you"ll gonna realize that the happiness you are chasing is within you. And when you start there, you'll gonna know that somehow you can be happy just being with yourself.