Hmm. You might be able to talk about diary privacy without bringing up your feelings for the girl. But there is the possibility that she'll say "Yes, I'm sorry, I did read your diary last week, and I want to ask you about what you wrote", and then there's the uncomfortable conversation.
If she asks, you could say something like "There are things I'm trying to sort out in my head right now, and I'm not ready to talk about it." Even, "Please don't ask. I need some private space in my head to figure things out, and it makes things harder if people are watching me too much." That's probably a pretty awful conversation, but it also might clear the air between you, and give you the space you need.
You're probably the best judge of your relationship with your mom. Have the conversation if you think it'll make things better between you, but not if it'll make things worse. It's hard for me to say without having met your mom. How calm and supportive a person is she?