This is just a different type of religious thinking, along the same vein as faith healing.
The universe isn't Uber Eats. You can't order something and it will somehow land in your lap.
Millions of really good, kind, happy, positive, well-meaning people live and die in horrible conditions - poverty, disease, war, etc.
To believe in manifesting is to believe that these people weren't good enough for peace and riches. That's a really shit way to view the world.
The better way is to accept the dumb luck and circumstances in your life, work at bringing more good into the world than existed before you came along, and seek contentment. Happiness is fleeting, but contentment can last a lifetime.
Look into Stoicism. That's a solid way of thinking that won't steer you wrong in life.