What helped me let them go was realizing that nothing is lost if you have the attitude of seeing them through a trial-and-error attitude. In learning any skill, there is time to learn the difference between what works and what doesn't. Life is the same. Nothing is a waste, if you choose to see past negative experiences as the means to grow into a more peaceful and prosperous person. The truth is that nothing is good or bad unless you choose to see it this way.
It helps to never blame the world or the people in it for your experiences because everything is a personal choice whether you realize it or not. When you take responsibility for your experiences, you access the mental power to see the hidden elements that sabotage your life.
The only failure you will ever have is when you do not take the time to find out what attitude caused the failure and make a commitment to change it.
Watch a fly fisherman. He will try different lures until he finds one that attracts the trout. He doesn't give up just because one fails to do the job. He uses trial and error until he finds one that works.
You do the same. Give thanks for the opportunity of finding out what does and does not work. Use your so-called failures as the means of growing into the being you are meant to be. Focus instead on what you want because, in this way, you will find the right mental bait to attract it.