Happiness can be achieved through actions. If you are sad now, it means you need to do something, it means you need to move and do something that will put a smile into your face.
But you still need to be responsible for what you are doing. Just because it is fun doesn't mean you are happy or you will still become happy later.
Do what will make you happy now and what will make you happy again later. For example, if you will exercise now... it will make you happy later because of the feeling that you get and the effect that it causes to your body. You will be happy because you are moving and at the same time you will be happy again because you are getting results. The conclusion is... you will become happy twice.
If you will party now even though you have an exam tomorrow, you will just be happy in the beginning, in the end you will feel bad because you don't know what to answer in your exams.
If you will work hard now, you will become happy because you are seeing yourself progressing. And then you will become happy again later because you will see the fruits of your labor. You will earn a lot of money, you will be able to buy the things that you want. It is a win-win situation for you.
But if you will just watch a comedy show now for 9 hours straight, you will only become happy while you are watching. After the show, now what? of course you will feel bad and stressed because all of your pending tasks were getting bigger and bigger.
Happiness will multiply if you will do the right things, it will change if you will make bad decisions that you knew from the start will not make your life better.