General:
* Dropped the ball: This implies a failure to perform a task or to take advantage of an opportunity.
* Missed the boat: This means you failed to take advantage of a chance or missed an opportunity.
* Screwed up: A more informal way to say you made a mistake.
* Botched it: This implies that you did something badly or failed to do it correctly.
* Went off the rails: This suggests a complete loss of control or a significant deviation from the intended course.
More specific:
* Fumbled the bag: This implies a lack of preparation or competence, leading to a poor outcome.
* Cracked under pressure: This suggests that you were unable to handle a stressful situation.
* Made a hash of it: This means you made a mess of something, often through inexperience or carelessness.
* Took a wrong turn: This implies making a mistake in judgement or decision-making.
The best idiom to use depends on the specific context and the level of formality you want to convey.