Sin is human insofar as we have the capacity to chose the evil over the good. But can you really sin if you are unaware of what the evil and the good are in a situtation? For example, take a 4 year old who just hit his sister in the nose because she took his toy. Does the 4 year really understand why his action was wrong? If he doesn't understand that then can we truly can that a sin? Thus you question of, if we sin and know we are sinning why should God even forgive that? Basically in order to ask for forgiveness for a sin we have to know that we chose the evil in the first place over the good. Human beings by nature tend towards the good but there are moments we chose the evil over the good for reasons that are natural to ourselves. We reason ourselves into the decision or perhaps let go of ourselves in the moment and just commit the evil. There are many elements that can bring us to perform the evil over the good, and what's worse is when we take an evil and turn it into a good therefore reasoning to ourselves that it isn't a sin after all.
That passage I find a bit too new-agey. Forgiveness is acquired, yes. Now this idea that the mind doesn't sin is a bit sketchy. The intellect doesn't sin, yes. The intellect doesn't sin because sin requires an act as it is an act against God or self or others. Thus sin is in the will. But this idea that sin is something you taught yourself is invalid. If sin were that, then sin is relative but we know that sin isn't relative as sin is due to internal AND external forces. Sure some people might view certain sins not as sins at all, for example a person doesn't see abortion as a sin, while another does. Besides how do we know we have sinned if are sins are self defined? Why can't I just self define the sin as something good? Forgiveness isn't something learned, it is bestowed. What is learned is something that might help keep you from sinning again. Words from a priest, a parent, a teacher all help bring the sinner back to following the good. Also the idea that forgiveness through God helps us forgive the sin we "think" we have made is again relativistic.