Keep in mind that the fruit they ate was the fruit of knowledge - the knowledge of good and evil. Prior to that fruit being consumed, that knowledge did not exist in Adam and Eve, and there
was no sin. Once they ate it, they became aware of what was good and evil, introducing evil into humanity that did not exist before. It was an act that could not be undone, evil now existed in men. It wasn't just one little sin, the act of eating from the wrong tree. There were much greater implications and consequences that came from that act.
They also committed the sin of pride by eating from the tree, trying to become like God by doing so.
Genesis 3:5-65 For God doth know that in the day ye eat thereof, then your eyes shall be opened, and ye shall be as gods, knowing good and evil. 6 And when the woman saw that the tree was good for food, and that it was pleasant to the eyes, and a tree to be desired to make one wise, she took of the fruit thereof, and did eat, and gave also unto her husband with her; and he did eat.
See, that's the thing though, their sin wasn't just eating a piece of fruit. Their sin was bringing sin
itself into the world. There wouldn't be sin in the world at all if not for that one act. Looking at it in those terms makes it easier for me, at least, to understand why it wasn't something they could just be forgiven for, because millions of people who came later are also sinning because of their original sin.
I don't know if that helps explain it at all - I'm sure you knew everything I said anyway. It's just a perspective I try to keep when I'm having these questioning thoughts, and it helps me understand it a little better.