I say this: Just let them meet you and get to know you. They will notice if anything is different about you.
If they ask about any medical conditions, then disclose. If they don't ask, then don't disclose. There are lots of people who have minor limitations like you who were born with them. This is no big deal. I think you are making a big deal about of it.
They won't just make you a nun instantly. They will want to spend lots of time getting to know you. They will see any limitations in how you function.
I see no need for you to make a big announcements such as, "Listen up everyone! I had a Traumatic Brain Injury that limits my functionality! Okay? Did everyone get that! I'm disabled!"
No. No need to do that. Just let them see how you do function, I think they will just assume you born with certain limitations, and they will decide whether you fit into their community or not. LET them decide.
But, even more important, TRUST GOD.
TRUSTING God means KNOWING that NOTHING can go wrong in your life as long as you keep your heart pure and stay in God's graces (as best as you can).
That means that even if EVERY religious community on earth were to reject you, even then NOTHING at all has gone wrong, since you will only have seen God's glorious Will work out.
God is constantly arranging the best possible outcomes for those who love Him. See? NOTHING can go wrong. NOTHING!
Your vocation is to love and serve God. The rest is all details.
She who loves and serves God will be loved, embraced, saved and sanctified by God and will be happy in this life and forever in the World to Come.
For those who love God, there are no accidents, no missteps, no errors, no wrong paths, no dead ends, no injustices. God is love. God makes everything happen that needs to happen for the welfare of his children who love him. Jesus said that God knows about every sparrow.
This is OUR FAITH. We don't need to fret and worry. Nothing can go wrong at all. That's what Jesus said. “Therefore I tell you, do not worry about your life." Matt 6:25
To be honest, I would not admit you to a religious community right now. Why now? Not because of a brain injury. But because of your lack of FAITH in God's providence. Faith means trust. Are you trusting God now? I don't think so. Your post title is "Afraid of being rejected." So, by your own admission, you are AFRAID. What are you afraid of? I think you are afraid that YOUR WILL will not be done. A person who takes religious vows must want GOD's WILL to be done above all. What you want and love and like and hope for does matter. God loves you, and sees your hopes and dreams. I don't say that your hopes and dreams are nothing. God wants you to be happy and feel loved and important in the right sense. But, any sort of worldly fear IS a terrible impediment to progress in the spiritual life.
On the other hand, gaining CERTAINTY in the Goodness of God is a MIGHTY uplift to progress in the spiritual life. Such certainty CAN be gained. I'm sure you know or can find the path.