Most fit problems are not about having the wrong size. They are about how ready-to-wear clothing is designed versus how your body is shaped. The good news is many of these are fixable without buying new clothes.
The Gaping Button Problem
Your shirt pulls open between buttons? That is a sign the chest is too small, but taking a tailor a size up everywhere else might be too big. A tailor can add small darts or snaps between buttons. Costs about 15 dollars and completely changes how the shirt wears.
The Pants Waist Gap
Your pants fit in the hips but gap at the waist? This is extremely common and easily fixed by taking in the waist at the center back seam. Takes a tailor 20 minutes and makes your pants actually comfortable.
The Too-Long Sleeves
You sized up for shoulder fit, now the sleeves are too long. Get them shortened to hit right at your wrist bone. This small change makes everything look more polished and intentional.
The Baggy Knee
Pants that bag at the knee usually just need a slight taper from knee to hem. Not skinny—just removing the excess fabric that pools. Makes a huge difference in how pulled-together you look.
The Dress Length Problem
The dress fits perfectly but the length is wrong for your height. Hemming is basic tailoring and changes the entire proportion. Do not suffer through the wrong length when this takes 30 minutes to fix.
The Tight Shoulders
Jackets that pull across the shoulders need to be let out or you need a size up. This is not fixable if there is no seam allowance, so this one might mean replacement.
The Armhole Fix
Tight armholes restrict movement. A tailor can sometimes lower them slightly for better range of motion.