Through writing you not only learn who you are but also make new knowledge that is important to the community at large.
You are no longer practicing with the already known, as you likely were in secondary school. You are being invited to join the fellowship of minds that are committed to voyaging into the unknown--to making discoveries, to finding new meanings, to broadening the scope of human understanding. Sound critical thinking, expressed in writing and helped by it, is the coin of this realm.
To participate in the conversation of the intellectual community, you must write. And the dialect of this community is one that will require some growing. You will be forced to stretch linguistically, not simply because you will be learning new vocabulary and concepts but because the thinking and logic which you now express will be more sophisticated and rigorous than what you are used to.
Stick with it. The more you write, the more comfortable you will become. You will enter the conversation, and your written products--as communications from scholar to scholar--will advance the learning of others, professors and fellow students alike.