You are correct. At the point of death, each of us will receive our own particular personal judgement, at which point we will go to either heaven, purgatory or hell. Those who go to hell will experience eternal punishment; those who go to heaven will experience eternal bliss; and those who go to purgatory will experience purification which will eventually lead them to heaven. I have answered this particular aspect quite a few times already so I won’t elaborate here.
So why the need for a second final judgement? There are a number of things God still needs to fulfill that cannot be done at the time of our own particular judgement. First of all, God needs to bring the current creation to its consummation so He can usher in the new heaven and the new earth. We don’t know for sure what this means. Some associate this with the final disaster that destroys the universe. Some emphasize more on the glorious coming of Jesus the King, or both. We were told not to worry, just stand firm in the Lord and be confident in His mercy.
There is also the resurrection of the body. So at the final judgement, everybody will be brought before God, everybody who already entered heaven or hell to receive their bodies back. There will be purgatory no more. Those souls will be emptied out and, too, receive their bodies.
Then they will all understand from God everything that has taken place from the beginning of creation to the end. They will see all God’s actions of mercy, and all the actions and intentions of each human persons will be laid bare, so that all will see the justice of why the wicked were condemned and the just received mercy. The wicked will then justly received eternal punishment with their bodies. The just will enter into glory with their bodies and rule over the new heaven and the new earth.