I've never been a parent, but it sounds like this is not necessarily something being a parent prepares you for. Childhood is a time when the foundation of a person's nature is solidified in stone and cannot change much after a person's late 20s (although God can do anything).
One of the most important parental activities is to let your children see your faith in God: He is in control and will work everything out for us (Romans 8:28). All we need to do is walk with him in his ways. Don't worry about the future, each day has trouble enough of its own (Matthew 6:25-34). God is in charge of all of our futures, not us, and he loves us enormously (John 15:5). We can't control our futures (James 4:13-15). We have every reason to be at peace, thank and rejoice in the Lord, trust him, and entrust ourselves to him.
Another of the most important facets of being a parent IMO is to do everything possible so that your children experience your love for them. When the storms of life come, along with unbearable pain, what they have deep in their soul is what is going to make it possible to keep going on. It is what is going to make it easier for them to learn to trust God.
I would go to your daughter immediately and tell her that you love her more than she knows, that you would do anything for her, and that whatever her sexual orientation is, it will not alter your love or acceptance of who she is for her even the tiniest bit. It will forevermore be a joy whenever you get to spend time with her.
I wouldn't let it sit until later, because it leaves a conflict internal, when the truth is such that there shouldn't be any conflict at all. She needs to trust that sticking with God means he will help her with whatever troubles she faces in life and will even deal with some of them himself so she doesn't have to. She needs to present her requests to God and persist in asking, and God will answer her prayers. She can trust that if she is fully willing to yield to God and his commands, she will remain in his will and will be doing what is right and pleasing to him. God sees her heart (attitude, motivation, intentions, desires).
Thank her for the note, because you want to share her troubles with her, because of your love for her and you have more experience trying to deal with the difficulties of life, even when you don't know what to do. It must have been difficult for her to even write the note (with talking about it to your face being even more difficult).