Just practise. There's probably no specific thing that you're doing wrong that can be fixed with a quick comment. Chances are you're doing everything wrong.
A friend who is a very high level, expert surfer, sailor, paraglider, skier, climber, swimming instructor, and former gymnast. I taught him to kite and he got the basics straight away. It still took him a month of regular sessions to break it and get comfortably riding. He would generally do the same wrong things over and over, or find new ways to mess up.
Having said all that, beginners make a few fundamental mistakes all the time:
1. Trying to get on the board before they have developed their kiteflying skills. If you can't fly the kite almost automatically there's no way you are going to be able to keep it under control while stuffing around with the board.
2. When water starting head off more downwind and try to get some board speed up before heading upwind. Not dead downwind. Say 2/3-3/4 downwind.
3. Almost every kiter keeps the bar pulled too far and in and the kite half stalled, especially when the kite is downwind and at the bottom of the window.
4. Most newbies ride out 50-100m, fall off, body drag in then walk back to start again. In an afternoon they may only get 500m riding. Find a spot with a lot of space downwind and a lot of beaches downwind. Once you can ride a bit and not crash your kite too easily, and relaunch reliably you are better off staying out and riding back and forth. You will get at least a couple of km riding and could get 4-5km. You will end up way downwind but it is better to ride for an hour, pack up and walk back 2-3 km.