There are 2 main types of stammer, an overt stammer e.g. were sounds are repeated , prolongation of sounds or blocking where a stammerer cannot get articulate the word.
Also you can have a covert stammer, this is were a stammerer will change or substitute words he or she thinks he will stammer on for words they feel they speak. I know a few covert stammerers and it is very difficult for them because they are continually thinking ahead when they speak, trying to avoid feared words, and it is mentally very tiring.
I am an overt stammerer.
I am a covert stamerer. I am quite good at it too. Hardly anyone apart from my missus and family know I stutter. You're right though, it is very tiring constantly thinking ahead.