What should I have done when my colleague refused to move from my previously occupied spot, and was my reaction correct, or does it indicate a weak personality?

The matter is easy, and you did well by not quarreling, for forgiveness and pardon are the virtues of noble and strong individuals, elevating the status of their possessor. This is confirmed by the saying of the Prophet, peace and blessings be upon him: "Allah increases a servant in nothing by forgiveness except honor." However, you should have gently explained to her the etiquette of gatherings according to Islamic law: if a man leaves his place and then returns to it, he has a greater right to it. You should also avoid sitting between two people without their permission, as the Prophet, peace and blessings be upon him, said: "It is not lawful for a man to separate two people except with their permission," unless it is in the mosque and there is a gap in the row.