Well, we are sinners because we sin. How is that hard to understand?

A sin is something you commit. If you were a sinner even without sinning, the word "sin" would not make much sense anymore, don't you agree?

And yes, of course Jesus's sacrifice was for future sins also. Or do you seriously think Jesus has to reappear every 100 years to be sacrificed for those who sinned inbetween? Jesus' death was a sacrifice by God to us. Jesus was sacrificed to compensate for all human imperfectness that causes us to sin. By devouring His flesh and blood, we accept the sacrifice and don't blame God to have made us this imperfect.