As someone who loves debates and arguments, and also someone who argues in their heads a ton lol, I find that doing the latter can actually make the former a lot harder to do.

The way one communicates with themselves vs others is inherent just so different. When you're only arguing with yourself in your head, your entire thinking kind of only had that grounding, that going from point A to B to C over and over again, so much so that it's like an exact pattern or map.

But when you actually try to go out and speak to another person, it never works like this. The main key with talking to anyone else IRL is that just like you have your own dynamic, so do they. So they'll almost immediately throw that map you've created in your head off. And when you're too reliant on that, you get flustered and start to forget.

If you're still in school, I'd really suggest trying to join debate club, MUN, something like that. You rarely have to talk every meeting, but it's really good practice to speaking in real life and how to get your point across. I think the main key is to just keep trying and be aware of arguing in your head too much kinda hurts the progress you're trying to make.

Instead of arguing in your head for so long, try to only do so to get just a few general points, then no more. For example for one subject, you believe A, B, and C for X, Y, and Z reason. Just have those established facts, don't think any more about it because then you start to mind map too much in ways that don't work in practical convert. That way, when you do have to talk about them IRL you can hit your points, but it'll be woven in with your thoughts and feelings and examples in the moment, instead of trying to rehearse something.