It seems (at least from my own observations) that doubting and second guessing, makes us look and be perceived as less confident. While telling the truth does not correlate with other people's perception as doubt does, removing doubt and second-guesses, works on our own perception of self-confidence, which then in turn, will result in being more confident in front of other people. In my opinion it is our perception of self-competence that needs to be overcome first, than the rest follows. So, when we tell something that we are not sure about, we internally doubt ourselves and our confidence. 

In reality, most things are not binary, and we should be aware of this and all the nuances rather than judging in absolute terms. Sure, this does, to some degree make us come across as unsure, or rather, less confident. But, as I said previously, confidence in my observation, is mostly an internal conviction about your actions and words, followed by outwards perception, which again is dependent on your own conviction. So, when dealing with nuanced issues that are harder to be generalized into binary truths or lies, convince yourself that issue is complex and nuanced not binary opposites.

Confidence is about your internal conviction about the nature your actions and words, as true or not true. 

It will then affect the way that you express yourself; doubts and second-guesses, will make you come across as less confident, which will be reflected outwardly. 

To avoid this, make it a point to avoid saying anything that you're unsure or have doubts about, while saying only what you are sure to be true. 

This in turn will make you feel more confident in everything that you say and do, and will be reflected on your outward perception. 

Confidence will act as a reinforcing feedback loop.