Back the wrong horse
Definition 1 of 1
Pro Tip 1/3Metaphorical Use
This phrase is used metaphorically to indicate a poor choice, rather than literally betting on horses. 
I feel like I backed the wrong horse by choosing the slowest queue at the supermarket.
Pro Tip 2/3Negative Outcome
Use the phrase when the chosen option results in a negative outcome or failure. 
Everyone said I backed the wrong horse by investing in that company just before it went bankrupt.
Pro Tip 3/3Decision Regret
The phrase often implies regret over making the wrong choice or decision. 
He backed the wrong horse by moving to a new city that he ended up hating.