Rainbow Dash had a normal fillyhood growing up, at least until the day her father died. she blames herself for it, saying things like “if i hadn’t asked him to go out to the store that day, he’d still be here” and “it’s my fault! it’s my fault he’s gone!! don’t you get it?!”  
after she grew up some and moved to Ponyville to join the local weather team, she discovered hard cider. she drank it a lot to drown away her worries and fears, and more often than not she ended up going to work drunk. some of the weather team noticed this, and pulled her aside to get her the help she needed. she did okay after that, and even ended up getting into the Shadowbolts (my AU’s version of the Wonderbolts, for reasons to be told in the future) after she met the other Mane 6! but then, her brain relapsed and she spiraled into a deep depression and blamed herself all over again. Rainbow has started to return to her drinking habits again, and she often visits Applejack when she’s drunk, so she can cry and vent to her friend. hopefully, AJ and the others can help her get through this, so that she can return to being a Shadowbolt and get back on her hooves.