How Do I Let Go??

Have u ever been with someone that ur madly in love with (live with as well) had plans on marrying but then discovered theyre not who u thought they were n u kno for a fact that it cant work after they've shown their true colors or addictions (the mask is off) the person u fell in love with is not who u thought they were red flags every where but u want the old person back....How can u or did u just walk away?? I wanna let go cuz i feel it can be fixed but maybe im not being realistic