What should I do if I am not sure whether I have committed a sin or not? I already go to confession once a week usually, but should I go a second time? I really am not sure if what I have done is a venial sin, or mortal, or not a sin at all.
What you are describing here are classic indicators of a scrupulous conscience. Scrupulous people see sin (or the possibility of sin) in everything, and are tormented by thoughts that they “might” have sinned. In such a case, you should only make a special trip to confession when you know with certainty that you have sinned mortally, and you may even want to reconsider how often you ordinarily go to confession. I recommend talking to your confessor about it, but the usual recommendation for how often a Catholic should go to confession is about once a month.