Wow, it sounds like you are on the right track since you desire to support your child. Sadly, you are in a bigger minority than we are!
I am confused, however, by some of your terminology. Is your child a transitioning transsexual woman? Has she been properly diagnosed by a gender trained psychologist yet? If not she may be self medicating the hormones. This is deadly serious. Self medication can be deadly. Please ascertain just what is going on there. Her life may depend upon it.
Now assuming she is a TS and is being monitored by a medical Dr. then the next issue here is your religion not accepting her.
You are quite right. The Catholic church is legendary in not accepting change. It took them until the 1800s to accept the world was not flat! For them to accept the truth about a transsexual person will not happen in my lifetime or generations to come. They simply do not do science and education well. The church will spread lies about her and try to do harm to her in any way they can. Count on it! She will never ever be able to marry in the Catholic church. They will not amend her baptismal records to the corrected gender. They utilize antiquated logic that is mostly iron age mentality. They will do anything the can to destroy her and likely you too for being a good parent and loving your child.
Be advised the Catholic church has a hate agenda!
What can you do to help her and make her more comfortable as her true self with you? As others have said ditch the male pronouns permanently. That is a very painful thing to a transwoman. Educate yourself and read up on the subject including civil rights battles going on. Remember in most states she has no civil rights whatsoever. She is open to so much narrow minded ignorance and it can all hurt her. Read books like "True Selves" By Dr.Mildred Brown.
Join Pflagg if there is one near you. Look into support groups for her and also for yourself. Stay away from Catholic Social Services. They hurt people a lot!
Remain a loving parent!
Sharon