What are you going for an interview for? If its for a job where youre going to be surrounded by people, then obviously they are going to judge you, and its going to be more difficult. If youre going for something like a uni interview, try and put a positive slant on it. For example before the interview starts explain to them, and however long it takes, dont worry, that you have had a speech impediment since you were younger, and it gets worse when youre nervous. Obviously youre really nervous at the interview because you so want the place, because of (insert reason). Then put a positive spin on it. For example, although it can have a negative impact on your life, its also had some positive ones, for example you find it easier than many other people to understand people with other problems, having spent your own life trying to deal with this.
I know this doesnt help with the actual speech impediment, bu it may help take some of the pressure off, or mean that the speech impediment has less of an impact on the interview.