Justification happens once and is sudden. (Salvation in trusting Christ alone for the forgiveness of your sins. Knowing that Christ paid the judgement for your sins.)
Sanctification is on going and won't end till heaven. (It's the sandpaper on the areas that displeases or dishonors God. The growing into Christ-likeness process)
Just because you are justified, doesn't mean you won't struggle. We're all in being set apart and sanctified for His glory!