Thank you for all your replies. Very helpful.

I think that by looking at her and she catches me looking, I am afraid that I will reveal something to her - maybe some feelings towards her that I am not allowed to have. I think the feeling is mainly that objectively I think she’s pretty, but to me, even that isn’t allowed.

I dont think I could tell her exactly that.. but I think I could start with ‘I’m afraid that I’ll reveal something that isn’t allowed.’