It might be also that as a child you weren't taught what honest & sincere looked like from your parents if they weren't a good example of it, or if you didn't live around good examples of it from other family members & if it wasn't really talked about, you wouldn't have really had the chance to learn what it's really like to be honest & sincere.....thus questioning whether you are or not at this point in your life.

I grew up around parents who were socially awkward & a mother who had really low self-esteem. Even though I didn't have the problems they had, I was very unsure of myself in social situations even though I got involved because it was NORMAL for me, I always questioned how I acted & whether it was acceptable of not.....I learned from observing people around me & comparing myself.....that works as long as those around you are not dysfunctional & not giving you a good example....& with honesty & sincerity, it's a lot more difficult because it's not an outward behavior like actions are...it's inward thought processes that you can't always tell.