Can't think before talking or I stutter?

Can't think before talking or I stutter? Hi, I've always had a lisp and would sometimes mispeak but it was never a chronic stutter, I would always stutter extremely badly whenever trying to read outloud in school, to the point some of my early teachers thought I was near illiterate. More recently I've realized, whenever I try to record a Youtube video I just can't I have an idea of exactly what I'm going to say then as soon as I start to say it I just get stuck on letters, repeat words, or forget words making it impossible to understand. This really irked me because I was trying to record a video introduction for a job interview that they requested- it was supposed to be a 1 minute video but it took me almost and hour and a half to film, and I'm still not happy with the result. I understand that I'm just describing a stutter like what everyone else here has to deal with, but what I don't understand is that it only happens when I am thinking about what I have to say, not speaking off the cuff(while it still happens sometimes, but nowhere near as often). And, it is not nervousness I don't think, I can do job interviews where it is just a conversation fine, and I have recorded Youtube videos where I don't think about what I'm going to say before I do it and its also much better then. So, any advice or anyone with similar experiences?