Just when you think you have things under control, you're managing your life, you're managing your future, you're managing yourself and all of your idiosyncracies; you think you've chosen a path that suits you, that you can depend will carry you to the next chapter (one that you actually want to have happen); you think you can finally trust people and believe you have enough of an understanding of how they work and how to deal with them to know how to move forward.
Then you make changes to your life, changes that you believe are going to make your life better, you rearrange everything to be able to move along to the next stage of your life, you sacrifice those things that you don't need anymore shaking off the chains that bind you to your past self, you've made decisions based upon all available information, assumptions, presumptions and the faith and trust that things are going to go (with a small margin of error) as you expect them to go.
And then you get fucked. The reset button. That innocuous little button that is usually only hit on accident that is typically just a "oh, well, shit" moment that you just move on from but, from time to time, turns into an uninstall windows, reformat your hard drive, left with the blinking fucking C:\> prompt.
It isn't what you've left behind that is missed, it isn't that you took your world and re-arranged it and it ended poorly that makes you sad, it isn't that your expectations weren't met that make you angry, and frankly it isn't that you have to start over from (what seems like) scratch that makes you unsure of your future, it's the re-visiting of those lost and alone feelings you thought you had left behind in the not-so-distant past and the self-doubt and self-loathing and the mistrust of your self and your decisions which you had believed you were if not at least aware of but also possibly comfortable enough with to be able to finally move yourself forward that hurts. That nauseated sickness under your stomach when you realize you will probably be alone for the rest of your life, that you have poor decision-making skills, that people around you cannot be the answer to your ills (and possibly can't be trusted with much of anything) and that you plainly suck at life and have to (again) learn to come to terms with it.
When you stumble and fall flat on your face, aware that it was your decisions that made you fall, it's hard to get back up and dust yourself off. The doubts, misgivings, lack of trust, hurt and fear surrounding the potential of the next failure (because it is inevitable) are an insurmountable looking mountain placed on your back.