When you serve, whether in a match or when practicing, you should always first visualize the serve in your mind. You should see the serve's entire path and everything it does before serving - the contact off your racket, its speed and spin, the first bounce, its path over the net (low, any curving due to spin), the second bounce on the far side, and where it bounces after that. You should also feel the contact in your head, whether a grazing contact to create great spin or sinking more into the sponge for more speed.
Then serve, and observe if the reality matches what was in your mind. This gives feedback so that you can learn to control and develop the serve. Most players who have trouble controlling their serve (i.e. most) do not use this feedback, and so it's no surprise they can't control their serve. This also allows you to improve the serve by visualizing as it should be, and striving to match that.
When practicing, perhaps play a game with yourself where you visualize different serves, and see if you can match what's in your head. Practice until you can do this with all of your serves, including both spin serves and fast, deep serves. Perhaps even put targets on the table and visualize hitting them, and then do so. Eventually you'll have the feel of any serve you can visualize in your head, and to repeat that serve you just have to match the feel of the serve that you've learned.