When you're depressed, it means you're extremely sad about something and it's affecting more than one area of your life.
Depression is in the mindset. If you can tell yourself to not be sad and to be happy instead, if you can tell yourself that you WANT to be happy, you'll likely end up being happier.
However, for many people, the depression has gone down so deep that the person doesn't even feel motivated enough to WANT to be happy.
So my solution is to time travel.
When you're being sad, it most likely has to do with something in the past or present.
Something that happened to you in the past didn't resonate happily with your being so you became depressed. Maybe you're not happy about your current situation.
So instead of thinking about the sad things in the past or present, think about the things in the future.
Think about your goals, what you want to achieve, what you want to do, where you want to end up. Think about your dreams and visions.
Then think about your current situation again. Is what you're doing now aligning with where you want to end up?
Maybe you don't want to be doing the things you're doing now, but you need to do them nevertheless if you want to reach your goals and dreams.
Have long-term vision and short-term motivation. -Tropical MBA
Sometimes, you need to sacrifice instant gratification to gain delayed gratification.
So in time traveling, there are 3 different time periods you can think about:
When you're sad thinking about the past, try shifting your mindset to something in the present or future, like your vision, dreams, or goals.
If you're depressed thinking about the future and what's to come, try shifting your mind to happy thoughts in the past, like your past vacations, or a time when you felt extremely elated.
I've discovered this technique on my own, and it may or may not help you out; the theory is still in its infancy and experimental phase.
Try this time traveling out! Let me know in the comments if it has worked out for you.