How do you fix disorganized and unstructured thought/speech?

If you go down the rabbit hole of STAR and other techniques that are recommended here and decide that they are too complicated or hard to execute, do the practical thing that you see politicians do in interviews and debates.

Pick some talking points you want to hit. Rehearse the absolute crap out of them and have them committed to memory. Then figure out transition sentences and phrases that lead into them.

“That’s a great question, it reminds me of the time that (TALKING POINT)”

“Good point, on the other hand (TALKING POINT)”

It doesn’t matter much what you’re transitioning from as long as it has a slight relation to your talking point.

By doing this you are controlling the agenda of the conversation and you can have a comfortable home field advantage.

This is how every politician preps for debates and it works.

As another user said, learn and practice methods like STAR or PAR.

More generally, learn a few storytelling methods in the context of coaching and personal branding. Write your own professional career story, then with each iteration make it shorter on the personal introductions, while getting to a few key points that you will throw during the interview when asked for details. Do not write those in the resume, those are your memory aid and cheatsheet to nudge the recruiter if they are interested.

Rambling and going on tangents, and then coming back could be a sign of lack of focus and the ability to stay on topic. If you have ADHD or poor sleep habits, I'd get this in check too. I tend to do this a lot when tired or when I haven't had my morning coffee. It's so bad to the point coworkers tell me "hey, but I asked you Z! Why are you telling me about Y?" And in my mind it makes sense, because I will eventually get to Z by wrapping up the story, but others are neither expecting this nor have patience.

Therefore, practice communicating your ideas more clearly in all contexts, so that when you are caught with your guard down, your muscle memory will at least retrieve a better, more concise answer. Good luck!

As someone who, myself, has very scattered speech and thoughts and yet still has to be able to speak in front of others for many reasons, I would say that the best thing you can do is to practice what's called "extemporaneous speaking." The literal definition is just "speaking done without preparation," but, as I learned from one of my prof's in college, it's more than that. You have a rough outline prepared, even some stories, maybe opening or closing points. Then, think about the kind of questions you might get asked. Don't prepare for individual questions, but prepare answers that could fit multiple answers with a bit of tweaking on the fly. Having a solid idea of where you're going is key. Admittedly, I still have trouble with this too, but I have far less than I used to. I wish you all the best!

There is not information, but from what I read, I would try the following exercise. Practice reading a couple of paragraphs of a text. Then, out loud, summarize those two paragraphs in a couple of sentences. Leaving something out is a necessity, then.

I find my speech to be far more fluent when I practice lateral thinking skills, like doing hard crosswords. I stop blanking on words, even those not in the puzzles, and zi can shift mental gears a little easier.

Most employers expect you to answer behavioral questions in STAR format.

Look that up. Write down answers to a few common questions now. And then just practice reading those answers out loud.

is it just in interviews or do you experience that all the time?

Get an editor.