Ensuring your meatballs stay together requires a balance between the meat, the binder, and your cooking technique. If the mixture is too wet, it is often because of high-moisture additions like grated onions or too much milk. You should ensure you are using roughly one egg and sixty grams of breadcrumbs for every five hundred grams of mince to provide enough structure.
Another essential tip is to chill the formed meatballs in the fridge for at least thirty minutes before cooking. This allows the breadcrumbs to absorb the moisture and the fat to firm up, which holds the shape during the initial sear. When frying, do not try to flip them until a brown crust has formed on the bottom, as they will release naturally from the pan when they are ready. If you move them too early, they will definitely break apart. I have explained clearly why the resting phase is so vital for meal prep.