it's complicated There are tons of factors that can influence skin conditions: anything in your diet, allergens, stress levels, the amount and quality of sleep you get, cosmetics, even clothing.
Try to get as many of these as constant from day to day as possible, then attack one group of factors. For example, two months ago I ran a strict Whole 30 trial and my skin cleared up to point where I and others notice (unprompted).
Once you get results, change one single variable at a time. For instance, reintroduced dairy for a week, but keep everything else (not just diet, but all other factors) the same, and see what happens. Nothing? Great - dairy is OK for you. Continue by reintroducing sugar, etc.