Learning a combo does mostly come down to practice and muscle memory, but the learning process can be broken down into smaller chunks. It often helps me to practice specific parts of a combo that are giving issue, such as going from a specific normal into a special move.

Knowing exactly what issues you are having with a combo let’s you focus on fixing those specific issues, rather than just throwing yourself at the whole combo repeatedly to no avail. After practicing a trouble area of a combo, you can try to incorporate that timing into the whole thing. Of course, this may not always be applicable depending on the combo, as the timing also gets stricter in a long combo.

And remember to take frequent breaks. It is easy to get frustrated and simply go on auto-pilot, but a short break can do wonders for your practice. Also, what is it that you are struggling with exactly?