Think before you speak, ok: preferably the day before. Think the week, month and year before - your whole life, as a general thing, think.
And after you've spoken with someone - think then! About what was said, and about whether what you mean has changed. Or feels like it needs to. And if it does, think some more. Think it through.
But don't think while you're speaking.
While you're speaking: focus on putting what you mean into words. Not on who you're saying it to, not on how different people will take it. Put what you mean into words. Work at this with focus and determination, all the while you speak. Don't distract yourself thinking up ways to soften, spin, or silence what you mean.
You know what you mean when you open your mouth to say it, right? Focus on that, and on getting it right. That's quite enough a task to set yourself, when you open your mouth. You opened it because you had something to say. Focus in, nail it - done. Then sit back ready to hear what the other's take is, and while you're listening - yes, think! Think then, about what they're saying, preferably. Not about what you intend to inject just as soon as you can finish pretend-listening and break in edgewise.
Listening thoughtfully to theirs can only help you understand your own meaning. Once you've digested what they have to say, consider your reaction. Now, your reaction can simply be thoughtful silence. It can also be a few introductory remarks noting where you agree with these points of theirs, before advancing on those other points. Your reaction can be to question, following on from their statements, seeking clarity. Exploring just where your grasps of reality overlap, and do not.
Or, your reaction may be to know precisely what you've got to say. So say it! Put what you mean into words.
You'll have thought it all through before, surely? But if not, even better! You'll have ample time to think it all through afterward, after all. From daring to speak, you'll gain more rounded insights on everything that comes up, wherever you've participated. You make a habit of this, you'll end up knowing pretty dang well what you mean on just about anything that does come up. Better yet, you'll know how easy it is for you to correct your views, using the best parts of other people's views - wherever they improve or improve on yours. That's a thing you can see for yourself, if you ever make a habit of focusing hard and sharp on what you mean, instead of on how to pitch it, twist it, cloak it in mist. Bury it in shit, or sink it in silence. The people who worry about that kind of stuff never learn to think clearly about anything.
The people who think first and shut up learn nothing but the inside of their own brains, and the cowardice that therein dwells.