Quick Tip to Maintaining Happiness
It's really simple - always have something in the short term to look forward to. Being excited about something yet to come and knowing you don't have to wait very long for it, is one of the best feelings in the world.
This can be anything that makes you feel happy, whether it's a material possession, or a day trip, or an activity. If there's something you want to buy, such as a video game or a book. Don't do it straight away, plan to do it 3-4 days away. Giving yourself 3-4 days of 'looking forward to it' really motivates you to make it through those days, and also makes you appreciate it more when the time arrives.
Visiting a friend or family is another idea. Whilst making plans does not always work out, making simple plans where you make the journey and the effort usually does. Day trips or meals or even making time for a hobby you enjoy. If you like drawing and painting, try and spend 3-4 days getting everything done so that you can spend an entire afternoon on your art!
TV shows and movies are another great way to do this. As many run weekly, you can find one you enjoy so much that thinking about the next episode to come excites you. (Game of Thrones recommended) Or you can create your own schedule, have a night watching cartoons, movie nights... whatever you like!
This is just a small thing that has worked for me. And I hope it works for you too!