When you put pen to paper and bring life to your design, you are making a commitment. Designing is a series of actions and decisions that you make to reach a solution. We are problem solvers after all, right? Our design is supposed to solve problems, be it communicating a corporate message or driving fundraising for a global initiative.
Thus, we as designers should be confident. Not confident as in “I am superior, and my work is perfection" or confident as in “because I made it, it’s awesome". I’m not talking about that kind of confidence. That is just being a self-absorbed jackass. I’m talking about a confidence that comes from making good decisions. It’s the same confidence that is manifest in buying a bike. You make a decision on the best bike for the money considering your research, budget and personal preferences. That decision was a good decision based on a series of good decisions.
In the same way, we make decisions as designers. We do our research for the client and the project, we have a budget that we either are given or establish and we have the client's and our aesthetic preferences that we will work within. From there, we make decisions to create a finished project. Hopefully—and this is the tricky part—our confidence leads to decisiveness and vice versa. One of the biggest disservices you can provide a client is a lack of confidence in decision making. So go and be confident in your work.