I found one of my pronunciation mistakes while watching the video (or: a video) that I just uploaded to YouTube.

It would be more natural to insert “thanks to the automatic captions on YouTube” at the beginning of this sentence:
“Thanks to the automatic captions on YouTube, I found one of my….”

I had been mispronounceding the word "image" as "immersion" all the time, but never noticed.

I’m not sure what you mean by “all the time” — throughout this video, or before this as well? If you just mean in this video, then the correction “the whole time” is good. However, I only hear “image” (or rather “images”) once in the video, so I think you’re probably also referring to times you’ve mispronounced this word before this video. If this is the case, then I’d change “I mispronounced” to “I had been mispronouncing,” as in my correction. “I was mispronouncing” would also work.

I mispronounced the word "image" as "immersion" all ththe whole time, but never noticed.

More natural sounding

I found one of my pronunciation mistakes while watching the video that I just uploaded ton YouTube.

Thanks to the automatic captions on YouTube!, I finally discovered that I've been making this error for the first time.

I found one of myut that I'd been making a mistake while pronounciation mistakesng something while watching the video that I just uploaded to YouTube.

I was mispronounceding the word "image" as "immersion" all the time, but never noticed.

Thanks to the automatic captions on YouTube, I corrected my mistake!