My absolute favourite part, of my absolute favourite movie, is the scene in Forrest Gump, where Forrest is running, no destination in mind, just running, because he felt like it, and he steps in shit. The man talking to him at the time yells out "Whoa Man! You just stepped in a big pile of dog shit!" and Forrest just keeps running "It happens".
The saying itself "Shit Happens" is an ode to the fact that things are indeed going to go wrong or go bad in your life, it's part of the contract we have with existence and part of the burden of living. Things ARE going to go wrong and there's nothing you can do about that fact but I like to add my own little part to the end of the saying.
"Shit happens, but you don't have to step in it" or some kind of variation of that (I change how I say it week to week to be honest with you)
So what does that even mean? Well, your metaphorical shit-that-has-happened is the things that are going to wrong in your life, speeding fine (shit), get mugged (shit), girlfriend cheats on you (shit) aaaand you get the idea? Right? So now the shit has happened and you have two options, step in the proverbial poop or just take a step to the side and carry on with your life. Stepping in the stinking pile can have many different meanings, but I guess I can sum it up by saying that negative reactions to already bad situations can and often do make things worse, social withdrawal because of a bad experience, holding a grudge that harms you in the long term with resentment, starting a fight and holding onto anger are all examples of "stepping in it".
Everyone has done this and everyone will, but at every point of time you have the option to; take off your shoes, throw them aside, and keep walking, un-step yourself from the shit and maybe even one day you will see the shit happen in your life, you can stop, observe it's density, colour and scent and then just keep moving without stepping in it (has the metaphor gone to far now?).