I've been having a little trouble lately.

MY GUESS: I joined a group English class on an app a few days ago.

ALSO POSSIBLE: I took a group English class via an app several days ago.

And I found that sometimes I mistakenly spoke out Ane word, when I actually wanted to say Ba different word, and I didn’t notice thatis until I left the class was over.

It’ was so annoying.

And I found that sometimes I spoke out A word when I actually wanted to say B word, andbut I didn’t notice that until I left the class.

Yours is OK, but two quick things would make it more natural:
* Normally we say "Word A" (etc) instead of "A Word" → I'm not sure why, unfortunately... normally the descriptor goes in front of the noun, just like you did
* "Speak out" is a phrasal verb that means something more like 發生/直言反對; it means to raise your voice, particularly in opposition of something that you disagree with. Here, you could say: "accidentally say" (seems a bit softer) or "blurt out" (it has more of a connotation that you are supposed to be quiet and end up saying something, but I think it's OK here, too!)

Also, you could optionally say "would say/wouldn't notice" instead of "spoke out/didn't notice". The difference is just that "would" makes it seem more habitual, like something that happened several times, and not necessarily on one specific occasion.

I want to avoid that.

I don't think "avoid" works perfectly here because "avoid" means to consciously not do something or intentionally take steps to prevent something from happening (The subways are packed during rush hour, so avoid commuting from 7–8 AM if possible; she took a detour to avoid the heavy traffic)

I can't think of a word to replace "avoid", though 🫠 maybe something like "I'll try to be more careful in the future."

And I found that sometimes I spoke out Aaid word A when I actually wanted to say B word B, and I didon’t notice thait until I leftave the class.

And I found that sSometimes, I spoke out Afound myself using word A when I actually wanted to say B word, and B. However, I didn’t notice that until I left the class.

I want to avoid that.

I can relate to this for learning German.

And I found that sometimes I spoke out Aaid word-A when I actually wanted to say B word-B, and I didn’t notice that until I left the class.

This is more of a stylistic feedback, so you’ll probably receive various ideas on how to convey this.

What you had was clear. My correction is what I’m accustomed to hearing in the northeast USA.

You could also say, “…sometimes I said X when I meant to say Y…”
Using x/y “variables” as placeholders is a bit more common in my experience.

In the class, I chatted with different people in English.

"a few different people" feels a bit more natural here.

And I found that sometimes I spoke out Ahad said word A when I actually wanted to say B word B, and I didn’t notice that until I left the class.

And I found that sometimes I spoke out A wordwould say A when I actually wanted to say B word, and I didn’t notice that until I left the class.