You feel defensive in an argument because you are unsure of how strong your side is. You want it to be true without challenge, and you worry that the slightest bump in the road will send it into a ditch.
Take the time to understand the other person’s point of view. Really listen and try to empathise with what they are thinking and why they are thinking it. If it’s an argument you have a lot, make the effort to actually argue it (in your head or to someone else). Get really familiar with what it takes to believe what they do.
Now, when you you argue your genuinely-held belief you do so from a position of understanding both sides. You will find you are a lot more confident in your position.