To burn your bridges

to commit to an irreversible course of action that makes an alternative impossible subsequently.

e.g. My sister is very unhappy at work but her boss is a key player in her industry so it’s important that she leaves in a way that doesn’t harm her future prospects; she mustn’t burn her bridges.

The idiom reflects the practice of burning down a bridge after crossing it during a military campaign, leaving no choice but to continue onwards.