I hear some of my photographer friends talk about not having confidence from time to time. They question their knowledge and abilities. They are unsure of how to get the lighting effects that they want or a certain look or feel in a photograph. What I've observed most of the time in this situation is a lack of understanding of their equipment and how light works.
Most of the confidence problems can be solved by one simple thing... Practice! I'm constantly practicing shots. In the age of digital cameras it has become rather inexpensive to practice your photography. Grab your camera and a lens and go out and shoot. Shoot at different times of the day, different weather conditions, different places. I've gotten great shots at 12:00pm on a sunny cloudless day because I know what the light is doing at that time of the day and through years of practice shooting I know exactly what I'm going to get. You'll hear a lot of photographers say: That is the worst time to shoot, the light is too harsh. A lot of them will say never shoot people in that kind of light. They also say look for shade to shoot people in, Never shoot people in direct sunlight. That's crap. I do it all the time with excellent results, However I also shoot in the shade as well. I feel equally comfortable shooting in all kinds of conditions because I practice.
Another thing that can build your confidence level is knowing your camera. Explore your camera settings. Read the manual that came with your camera. Let me say that again, READ THE MANUAL. You will find all kinds of information about your camera. I learned about rear sync by reading my camera manual, I then went out and practiced with it to see what I would get. I now have confidence with it and I use it at weddings a lot to generate a sense of motion in my images.
Once you know how your equipment works, you've got your lighting down go out and experiment. I'm constantly experimenting with my equipment. I'm always trying new things with it, new lighting setups, different camera settings. Always experiment. I sometimes take the same shot over and over again with different settings just to compare the differences.
Don't be afraid to screw up. I do it all the time. I learn more about photography when I make a mistake than when everything is going my way. One nice thing about digital cameras is you can screw up and immediately see the results on the back of your camera. Sometimes a screw up turns out great and it looks like you knew what you were doing (make sure you note all camera settings and lighting conditions in case you have to replicate the "effect") so don't be afraid.
Photography is not easy, anyone that says it is, is not a photographer. It takes hard work and dedication. On the other hand it's not calculus based physics either. It takes time and practice to become good at photography. Take the time and the confidence will come.