The adjective diffident describes someone who is shy and lacking in self-confidence. If you are shy and have a diffident manner, you should probably not choose one of these professions: substitute teacher, stand-up comic, or lion-tamer.
Diffident can describe someone who is reserved and restrained. Some may mistake your diffident manner for coolness or aloofness. Although it may be in your nature to be diffident, you will find it impossible to remain so when you visit my family. They are a big, noisy, outgoing bunch and they will make you join in the fun until you let loose and open up. Don't say you weren't warned!