Fight with S/O

I had a fight recently with my boyfriend and he was mad and thought I didn't trust him, because I wouldn't tell him some personal stuff... I didn't tell him personal stuff, because the topic was uncomfortable for me and I didn't want him thinking differently of me either so it was something I wanted to keep to myself. He thought I didn't trust him when I know I do, I just like to keep certain things to myself.
The fight made me feel like a horrible girlfriend so I asked my sister for help and she said that I don't really love him and that I should be able to tell him everything. It made me feel even worse. The stuff I didn't want to tell was so personal to me that I I cry if I were to just talk about it. I feel comfortable talking to my boyfriend, but the topic it's self is something I'm uncomfortable with talking about to anyone at all. Am I wrong for this? What am I supposed to do?

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