We are dealing with two different things here, one is sin, and the other are the effects of sin.
Adam and Eve sinned, they went directly against what they knew to be the will of God.
The effect is that they where punished by being cast out of the garden and the loss of the gifts beyond those we are given at birth. We are given natural gifts, preternatural and supernatural gifts from God. (As a review, angels have preternatural gifts, or those beyond human perfection but still limited)
So too when we sin, even when we are contrite and forgiven, we still have the effects of those sins. We must atone for those sins, or the guilt derived from the actions. We need supernatural assistance to do so, as the offence is against God, therefore we cannot simply say “I’m sorry, here I’m making up for it” but rather must unite our actions with the Action of Christ and His Passion and death on the cross. If that takes place to satisfy the punishment due to our own actions on earth, (ie through penance or the proper application of indulgences) we can “go straight to heaven” as it is not the usual course, we are alloted by God’s mercy a place where we can satisfy God’s justice and be cleansed, or made ready to meet him face to face. (On another thread we can discuss particular and general judgement.)