Should I tell my boyfriend?

My boyfriend and I have been together for over a year and were sexually active. He's my first serious boyfriend. When I was 7 something happened to me that to this day I still haven't told anyone about. 
I had a friend who had a lot of family problems that were hidden very well and I stayed at her house a lot. My mom paid her mom to babysit, but most of the time the girls mom would leave us home alone.  One night she had a babysitter come over to stay the night. If I had to guess I'd say she was mid to late teens. In the middle of the night I woke up and the babysitter had her pants in my pants touching my vagina and trying to get inside. I pretended to be asleep and rolled onto my stomach so that she wouldn't reach in. I never told anyone and I refused to go back to that house. Now I'm late teens and this incident still comes into my head. I'm wondering if I should tell my boyfriend? I've never told anyone before so I'm not sure that I really need to tell him

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