Here's the thing: if you need "positive thinking" to be happy, positive thinking is not the answer.
If you need to "think positively," it means your default isn't already positive.
Positive thinking is a band-aid used to cover up internal emotional and mental dysfunctions. If you're not happy by default, there are some issues there. Baggage, unresolved trauma, things of that nature.
Instead of trying to positive-think your way out of being unhappy and unsuccessful, look at what's causing you to be unhappy in the first place. Once you find the source, it will look like a nasty thorn and you'll be a bit scared to pull it out. But once you do, that wound can heal and no band-aid will be necessary.
Positive thinking is one of those backward things where somebody noticed that happy and successful people think, do, and say positive things, so trying to emulate that is a surefire path to happiness and success.
If the happy and successful is a nail biter, should you bite your nails too?
If the happy and successful person jumped off a bridge, would you jump off a bridge?
It's backward, because saying, thinking, and doing positive things is a side effect of facing your demons and resolving those past traumas.
It's a byproduct of courageously going into, and then through, your inner darkness and finding the light on the other side.
Positive thinking is like a flashlight in a dark cave. It's useful for people trapped in caves, but you don't need a flashlight if you can find your way out. And the way out is through.