Use the tension in your hands, fingers, throat, neck - any tension - to tell you where to practice. Although many times you must push yourself to play or sing faster than is comfortable, most of the time you are to be physically relaxed. I don't mean emotionally relaxed, calm; I mean that there is no stiffness in your body. The physical places where there is tension and the things that you do that cause tension are the targets for repetition to create muscle memory and comfort in the physical mechanism - and that in turn produces great speed and skill. Look at your favorite amazing musician and notice how effortless their music looks.
A targeting sensor: vocally, the same sensor that makes you flat (this is hard, high) will tell you to support, listen and get it right.