Like the hair not the makeup

I would inform her that you have decided a different route for your make up, but would like to keep her on for your hair. Ask her how much the hair will cost alone. I didn't like my make up trial, so I scheduled a second trial. I like the second go-around much better.

Since you aren't going with a different makeup artist, it shouldn't be as difficult to hear. Just say you really wanted to do it yourself after thinking about it.

I originally planned to do a hair and makeup trial but told my HMUA I was unsure about having my makeup done because I often feel uncomfortable with a lot of product on my face. The whole point of the trial was to see what it would look and feel like and how it would photograph and wear throughout the day, and see if I wanted to have my makeup professionally done or stick to what I was comfortable with.

If you tell her you LOVE the hair but just didn't feel as comfortable with someone else doing your makeup as you had hoped so you just want to do it yourself, she'll probably understand. It would probably be a bit more awkward if you loved the makeup but not the hair, or wanted to go with a different pro for one but keep her for the other. Opting to do DIY on your face is pretty reasonable though!