General regret:
* As heavy as a stone in your stomach. (Focuses on the physical feeling of regret)
* Like a shadow clinging to your heels. (Emphasizes the persistent nature of regret)
* As sharp as a knife twisting in your gut. (Highlights the painful aspect of regret)
* Like a bitter taste lingering on your tongue. (Conveys the lingering negativity of regret)
Specific situations:
* For missed opportunities, like a wilting flower that never bloomed.
* For lost love, like a faded photograph that once held vibrant memories.
* For a bad decision, like a tangled mess of yarn that can't be untangled.
Ultimately, the best simile will depend on the specific context and the emotion you want to emphasize.