I have a speech impediment called Cluttering that makes this issue happen almost every time I speak :/
Sucks having to practice saying a sentence multiple times in my head to make sure I can accurately pronounce each syllable without slurring.
If anyone here has jerky/irregular speaking where it comes off as disorganized or rapid then you might have the same issue I do. Even though you can picture the exact words you need to say, you'll end up combining syllables together or have pauses during speaking where it sounds like you don't know what to say next.
I just try and prepare sentences in my head so I don't get messed up. One Cluttering thing i do when speaking is I will sometimes just revise the sentence I am speaking because the next word I was going to say was coming out wrong as I am speaking. This will just make me change what I plan to say and I will try and adapt the conversation on the go. It's when I begin to clutter more and more as I get farther away from the sentences I originally planned in my head.
I should also state it can affects your handwriting too. If it looks sloppy even though you try then that is also a symptom of Cluttering. The same issues with words not spoken right can happen when you write/type too. All my notes during school were horrid looking from this. I cant even state how much I had to rewrite this comment lol. Hope everything can make sense to you!
If you do have this then I am sorry for whenever somebody told you to slow down speaking.
Happens all the time. Thinking of a good joke. Trying to tell my friends. I fuck it up completly. My friends talk shit about me being bad at making jokes. Now a core cringe memory I will never forget. Love it