Usually, for a number of reasons, I don't cook fish (I know, it was probably ill-judged of me). What with my recently imposed dietary restrictions, I decided to give it another chance to compensate for a lack of meats I could eat. I bought cod, filleted it, put it onto my non-stick grill pan (which turned out to be not as non-stick) and grilled it for just a few minutes on each side. It was tender (so I believe I didn't overdo it), but I could hardly flip it or take it off the pan: it easily disintegrated, fell to pieces. Is it supposed to happen to cod? Was it bad cod (it smelled fishy – not spoiled, mind, but I heard fresh fish doesn't smell at all)? Was it a method of preparation that didn't suit that kind of fish? 
That's totally normal for cod, it's a flaky fish. The best way to keep it together is to leave the skin on, if possible. If the fish is skinless your best bet of keeping it together is to handle it as little and as gently as possible. The way I do it is to flip it once, starting it on the top first, then after a minute flipping it to the bottom, which is where the majority of cooking takes place. There are spatulas for fish which are longer to help support the fish as you turn it and keep it in one piece as you take it out, I actually use a crepe spatula for the same purpose. If you treat it gently and use the right tools and pans you can keep even flaky fish together. 