I'm going to tell the girl I'm not sexually attracted to her
Does she know you're not sexually attracted to her?
No, she won't take it well if I tell her. That's why I wanted to keep it a secret
You shouldn't mislead others by hiding the truth from them. Being honest may hurt her, but its better than leading her on. She'll eventually be able to move on and find someone else to be interested in.
I think she will be fine once I tell her the truth.
She will be and next time be more upfront with the next person you date.