The answer is definitely practice. There’s no easy way around it, man. Once you accept that, I will help you get the most out of your practice.
Experiment with how you hold it. Don’t worry about what it looks like or who else does it or if it sounds better than another way on paper. Only worry about whether it feels nice to you. Once you’ve found your most comfortable method of holding it, do not change to anything else, even if you can do one particular move more easily holding it another way. Very soon you won’t want to switch.
Practicing the movement slowly probably won’t help you that much… you’ll naturally tend to warp the movement as you speed it up anyways.
Do it full speed and watch your hand when you do it. Between the visual indication and the clicking of the microswitches, it usually becomes pretty clear-cut which part you’re missing; it’s worth emphasizing that part because you are very likely making the same mistake each time it doesn’t work. As already mentioned by others, games like Tekken, VF, and GG will have input indicators in training mode. This will further help you see what you’re missing if you still feel unsure.
If you have a PS2 with CCC2 or you’re comfortable with MAME, spend a little time with Ryu in ST. SF2 input is brutal and unforgiving for anyone who started on later fighting games. If you can learn to do his super consistently, I guarantee you will never have problems with any QCF or HCF input ever again.
When you’re practicing the movements, do it a few times until you get it right, and think about the feeling of the movement for the next few times. Then just do exactly that movement over and over a dozen times quickly without thinking about it too much. Repeat the whole thing again once. If you do this before playing three days in a row, I guarantee you will see a dramatic improvement with your execution consistency. Honestly. When you do it while you’re playing, think about your mistake but don’t think about the movement itself… just let it come out. It will feel natural in no time.
What kind of stick are you using? It’s worth noting that most people learning J-stick find that 2xQCF is the only input that is more difficult on it (as opposed to on a Happ or d-pad).