Hi, I'd like to help you with your question.
It is very common for those diagnosed with schizophrenia to refuse treatment. And even if they accept treatment, they often try to go off of the medications and stop treatment. So you are not alone in dealing with your daughter's situation.
Since she will not seek treatment, there is little you can do to force her. However, there is a lot you can do to empower yourself and find ways to help her, especially if she decompensates.
First, work on educating yourself about schizophrenia. It will help you recognize the subtle changes in her behavior/symptoms and if she starts to decline, you can step in to help. I've listed some books at the bottom of my answer to get you started.
Secondly, find ways to reduce your stress. Dealing with a loved one who has been diagnosed with a mental illness can be time consuming and energy draining. You want to help, but there is a frustration in the amount of help you can offer. So it leaves you feeling upset and stressed. Here are some steps you can take to help yourself:
Remember to take time for you first- you cannot be there for her if you are so stressed you can't cope. Take time away from the situation and get rest. Short vacations, getting your hair done, going out to dinner, or just even taking an afternoon for a favorite hobby can all help you to feel re energized.
Gather as much support as possible- this includes family, friends and support groups. You need others to be a part of the situation so you don't have to cope alone. Support can mean someone just listens to your frustrations or they can help keep an eye on your daughter while you are away for a bit. And don't forget to seek out a good support group to help. Contact your local community mental health center for referrals.
Consider therapy for yourself and other close family members- therapy can help you talk out your frustrations and feelings of helplessness with a professional who understands the diagnosis of schizophrenia and can help you cope.
To help your daughter, talk with her about treatment as much as she'll tolerate. Suggest to her to seek treatment not based on her diagnosis of schizophrenia, but because she is extra tired or other more basic symptoms she may have. That way, she doesn't feel like she is being labeled as "crazy" but really has a legitimate problem to go to the doctor for.
See if she will go with you or another family member to a doctor's appointment. And if you choose to see a therapist, ask her if she will go along to help you. If she sees that the therapist or doctor is non threatening, she may be more willing to go for herself. Clear this with the doctor first to be sure they know why she is along.
Encourage her to keep herself healthy. Keep the lines of communication open so she feels she can come to you if she starts to decompensate. These things will help her in case she does change her mind and is willing to seek treatment.
Here are some books that may help you with your daughter:
Surviving Schizophrenia: A Manual for Families, Patients, and Providers by E. Fuller Torrey
The Complete Family Guide to Schizophrenia: Helping Your Loved One Get the Most Out of Life by Kim T. Mueser PhD and Susan Gingerich MSW
You can find these on Amazon.com or your local library may have them for you.
I hope this has helped you,