I'm really sorry she read your diary, I can understand how you feel *hugs*
I think Whispering_Voices idea is good, you could also tell her (if you feel comfortable enough with it) that she can come to you and talk to you about it if she has any questions, instead of going to your mum instead.
You could use an online journal (there's one you can use on RYL, you can set it to private) to write down your thoughts as only you will have the password to acess it so it might make it feel safer to write in.
Take care
xx
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