SPOILER WARNER: I will be giving away many details of Albert Camus' L'Etranger today, so if you do not wish to know the ending of the book, please do not read any further.
I came upon an interesting theory as I was researching about existentialism for this post. It really delves deep into the fact that humans are fundamentally free to make choices. So free that sometimes, it is terrifying. The example I cam across looks at a person who is standing atop a cliff. That person may feel the fear of falling but also dreads choosing to throw him or herself off of the cliff. Basically, nothing is there to hold you back from choosing to fall or to stand, one way or another. This kind of freedom is fundamental and powerful, therefore terrifying. Connecting this situation back to existentialism, the truth is that every single action we humans take, is laden with choices such as these. We can blame the result on, really no one but ourselves. This is a hard concept to grasp and one that Meursault is able to, awaiting his day of execution. To put it crudely, the universe simply does not "care" about us. The world will continue to spin if we commit an error or if we do something awesome or if (like Meursault) we kill someone. Kinda scary. Actually really scary if you think about it too much. But I'll leave with a happy idea that stems from this kind of philosophy.
My parting thought: Don't take your mistakes so seriously. Enjoy what you have and don't dwell on the past. Another day will come and the earth will keep spinning, even though you erred. Embrace your life and forgive your mistakes. :)