What is original sin and why am I responsible for Adam and Eve's mistakes?
Original sin is a state of alienation from God, also described as a lack of original justice: that is, being friends with God. Because it is a privation of righteousness, it is not ‘a thing’ transmitted. It is rather an incapability to pass on original justice: you can’t pass on what you don’t have! (Catechism of the Catholic Church no. 404) So, although you personally, are not morally responsible in the same way as criminal guilt; you share with all humanity the lack of justice, but this is not the personal guilt of a long-dead ancestor such as Adam and Eve. This justice is restored through Baptism through Jesus’ Passion, Death and Resurrection (cf. Romans 6.3f).
God could have forgiven this selfishness and sin in any way he chose. But the way he chose to bring his forgiveness to us in the death of Jesus comes from his wisdom, and it deals with the whole situation of human sinfulness and selfishness, going right back to our remotest ancestors. When we act selfishly towards others, we distance ourselves from them, but we also distance ourselves from God too, who calls us to a life of self-giving love. In fact the whole human race is in a situation of being at a distance from God - from the very beginnings of the existence of each one of us, we are born into our race's situation of alienation from God. Death in itself is also our ultimate alienation from each other and from God. If the beginnings of the human race in our remotest ancestors had been different, God would have spared us death, and the inheritance of our race would have been entirely different. The fact that we all die follows from the fact that our race chose sin at its beginnings. Sin and death go together. We chose selfishness and sin, and we were not spared death, and we need to be forgiven and restored to life. But forgiveness is not simply a matter of saying someone is forgiven or deciding not to punish them. It means re-establishing the relationship and overcoming the distance and alienation between the two. So for God to forgive us means to come close to us once more, in our human lives and our human death, and to re-establish the relationship across the divide that we have put between ourselves and God. This even means that in Christ God even dies a human death - he comes so close to us in our worst moments - and then conquers that death by rising again. The resurrection then opens up new possibilities for human life - possibilities of selfless living, of death not as alienation from God but as a way of passing to a new life in God, made bodily at the resurrection. If we are moved to sorrow at Christ's death, this can move us to leave sin and death and selfishness behind and want to share in the new life of his resurrection.