The more you say Spells or Rituals out loud the more confidence you will have in yourself. Practice on your own. You will stumble, but you will get over it. Sometimes the lesson is to laugh at yourself.
Sometimes, I will say a spell three times for added energy. The last time you say it, say it with authority.
Spells can indeed be performed either way, however it does tend to be more advantageous to say the spell out loud if you are in a situation where you can do so. The human voice is a very literal process of manifestation. Your voice takes intangible thought and intent and idea and projects it outward physically in sound and language. The idea of speech within magic is the same. It is a direct and physical way of projecting your energy and intent outwards.