I literally trained myself to become a more positive person when I was 15. It wasn’t as hard as I thought it was going to be.
I set the goal that I wanted to be more positive. To me, that meant: complain less, see the good in people, focus on the good things in life, etc.
All I did was start paying more attention to my own behavior and thoughts, and make changes where necessary.
If I caught myself thinking a negative thought, I searched instead for a positive thought about the same subject. If I found myself about to say something rude, I asked myself if it really needed to be said, or if there was a nicer or more productive way to say it. If I felt miserable, I made a gratitude list or went on a rampage of appreciation.
Let me be clear: this does not mean that you bury/hide/ignore the things that are going wrong in your life, or the things that cause you pain. You can focus on those things, if and when your focusing on them will actually help solve or improve them in some way. If there’s nothing you can do about a shitty situation, though, you might as well put your attention elsewhere. We have a limited number of moments in this life; it’s silly to spend too many of them looking at what’s wrong with the world when you could be fixing problems or focusing on what’s going right.
You mentioned dressing brightly. That might work for you, or it might not. I like to do it because it makes me feel good when I’m in the right headspace for it, but there are so many other ways to “trick” yourself into feeling happier. You might want to try making a list of actions that make you feel good and then incorporating them into your life on a more frequent basis than you’re already doing.
But all that external stuff is just external. If you want to be a happier, more positive person, the real work will be internal: changing your thought patterns and your core beliefs. It sounds like a lot of work, but that’s only because you’re not in the habit of being positive yet. Being positive and being negative are the same amount of work; it’s just that most of us are more accustomed to one or the other, and what you’re used to is what feels easiest. Get into a new habit, make it stick, and it’ll start to feel easy.
More positivity resources: You Can Heal Your Life by Louise L. Hay. Ask & It Is Given by Abraham-Hicks. The Happiness Project by Gretchen Rubin. Lots of older posts on Gala Darling’s website. Pronoia is the Antidote to Paranoia by Rob Breszny. The documentary “Happiness.”