We should never dwell in the past. The past is over and, instead, we should look forward to the future.
Just get over it. Get over that mistakes you have made, get over that failed relationship. There are so many people that dwell on their failures, they cry about it and think about it constantly that it even affects the way they perform daily! There are even some who get so demoralised and beaten by their failures that they just give up.
Think about it, are you not wasting time by bullying yourselves for failures that already had its consequences played out? Are you not wasting time questioning your capabilities, thinking about whether you are the right one for the job?
Here is the thing, it does not matter if you failed an interview or made your boss angry, it does not freaking matter whether you have failed or messed something up because you were performing your best at that point in time! What truly matter is that you get over it quickly and learn from it if there is any value. Put in your best effort the next time round. Know that the world is not going to end when you failed because failing is a part of being human. Recognise that you can always push past your failures!
Time is limited and I personally do not think there is enough time to waste on feeling regretful nor feeling demoralised, instead, just get over it quickly.