@xtheconman - with regards to the human person, their forgiveness ultimately doesn't matter. If the person you sinned against never forgives you, but you never sin against them again, then you are forgiven by God for that sin. The problem in that scenario is that if the person who was sinned against commits sins in reprisal for the first sin because they couldn't forgive - they put their own souls in jeopardy, and there really isn't anything certain as to what, if any, responsibility you have from then on. A human forgiving a fellow human for something is, in the long run, for their own personal benefit rather than the sinner's.
Regarding repetition of sin: If you confess your sin in the sacrament and receive God's forgiveness, truly repent and sin no more, then God forgives that sin. If you knowingly commit the same sin again, can you say you actually truly repented and received grace? It's actually something of a theological...erm... useless discussion at this point, to ask, "Could God forgive Hitler for everything he did if he truly confessed and repented for his sins?" The doctrinal answer is "yes," but the practical considerations as well as our own human inability to truly forgive would argue every (meta)physical counterpoint to demand a "no."