Don't feel stupid. Anyone who criticizes you also has no idea how to box or shadowbox... and so they are in no position to criticize. Someone who does know how to box and shadowbox and criticizes you forgot that they too at one point could not put their punches together, their feet and hands weren't working together, and they too felt awkward around those who knew what they were doing.

So anyone who criticizes you is either ignorant, or forgetful that they were once in your shoes.