Most people who stutter start as children and continue into adulthood but if you can't remember stuttering as a kid, then you are obviously not in that group. It is very rare that someone suddenly starts to develop a stutter as an adult but it is possible. We call that "acquired stuttering". Acquired stuttering can happen after taking certain drugs, when someone has a brain trauma (e.g. concussion) or after a traumatic event. Missing and mispronouncing words is not a symptom of stuttering. Repeating words, syllables or sounds is. I would try to remember when you first noticed this, how it developed over time, how much you sleep and which substances you are taking that could contribute to this. If the problem persists I would see a Speech & Language Therapist for an assessment.