One thing that I've learned recently is that it's okay to second guess yourself. For me, this is something that happens a lot, but a normal human being is not going to be 100% confident in them all the time. Being unsure about something (or about many things) is completely normal, and after realizing this, I am much more at ease with myself.
It could be a choice as simple as what to eat for breakfast, or something as important as choosing the right college major - I have second guessed myself regarding both of these. It's hard to know if a decision is the right one for you, but you have to remember that it should always come down to what is best for you at that moment. I felt a lot of pressure throughout the past few years trying to decide what I really wanted to do with the rest of my life.
Questions constantly running through my head - Am I choosing the right career path? Do I really enjoy what I'm studying to pursue? How will this affect my future? - I wish someone would have told me sooner that these questions are nothing to stress myself out over. People and situations are going to test your confidence, but all you have to do is take a moment to assure yourself that in the end, what you are doing is what makes you happy. If you're not truly happy, it's okay to change your mind too. Changing your mind can be a beautiful thing. Without change, life would be pretty monotonous. Just remember that it's okay to be confident in yourself too.