When you believe something is possible, then you will try to do it. When you know something is possible, it will happen. How do you reach a point beyond merely believing, to a point of knowing?
The ultimate confidence comes from the doing, over and over again. Don't seek tricks or shortcuts. There's are no shortcuts, No mental or technological tricks. Just dig right in and get your hands dirty.
Soon enough, you won't just believe, you'll know with NO doubts. For instance, you may believe you can run a 200 mile marathon. Yet, only when you get up every morning at 5 am and run and work out, day after day after day, do you KNOW you can run the marathon.
The expression goes, seeing is believing. By the same token, doing is knowing. It is great to believe in possibilities. You can speculate on, and imagine, all sorts of wonderful things. And action is what will make reality out of all those possibilities. So jump in and take ACTION, turn your dreams into reality. You will then know what you're capable of accomplishing. And, knowing it with complete certainty, you'll make it happen.