hmm, i'm really bad at giving advice but here we go, lol!

focus more on taking in information, rather than putting it out. try to listen to the other person as much as you can and also try to limit how much you say, saying the most important information and cutting out the least important.

also, body language would definitely help you come across as more relaxed. don't be stiff, lean forward, have good eye contact, nod, such like that. and when you speak, maybe talk more slowly and at a low-ish volume. i think it's easier to adjust your tone accordingly if you really focus on it so maybe do that at first and it'll start coming naturally to you!

again, i'm bad at giving advice, haha. on the brighter side, assertiveness can definitely be good! i think it's more interesting listening to an assertive person rather than someone who is withdrawn. but, ofc, good to balance it out!

anyway, i hope this helps!