Another idea on how to keep being happy no matter what is to count your blessings and be grateful for them. Thank yourself, thank the Universe, thank the people around you, thank the things around you - there is sooo much we can be grateful for. I once watched a video with Joe Vitale, and it stroke me how positive this man is. When he was homeless, when everything seemed to go wrong in his life, hу heard that simple truth - "Be thankful for what you already have". And instead of thinking that this idea was insane for him, how could he be thakful for the situation he was in, he decided to give it a try. He looked around and saw a pencil. A simple pencil with a little eraser on top. And he started to express gratitude for that pencil. He was thankful for a PENCIL! - just think of it!
So look around - and see how man things you can find that you are thankful for. You do already have everything you need to be happy. Realize that and just be.