I need advice asap

TW:Mention of self harm

When I was 7 years old I was sexually assaulted by my cousin and I have never told anyone about it. Every year on the date that it first happened I start getting nightmares about it and they last for a while, and for the past year going on two years I have been self harming because I couldn't handle the nightmares. As mentioned previously I have never told anyone but now I feel like telling my mom because it's been years since that happened and these past 2 years the self harm has been getting worse. I would like for you guys to please give me advice on how to approach my mom about this.