I've got the same problem. I got my stutter just like you at around 17 and its worse around family and friends. I can talk freely with store clerks or doctors I don't know, but as soon I get to know people more, my ability to speak decreases and if anyone of them comments on my stammer it just vanishes. Sadly, I don't know any way to prevent this but I assume the cause is that you want to be more open and less planned in you speaking around people you know and like but the concealed and planned part of your brain tries to change your phrasing last minute but accidentally scrambles your whole sentence in the meantime.