Very angry witness

I guess all I want is advice, maybe a sturdy shoulder to cry on for a while... When I was very small I witnessed my mum get beat up by her boyfriend on a regular basis. Every day at school I was afraid Id come home to her dead and one day I got fed up hearing his voice. So I stood in between him and my mum, I remember my mum trying to pull me away but I was determined to stop him from hurting her anymore. Of course I was only a child and he didnt stop. It carried on till the middle of highschool. And Im sure going that long without help did something to my mind, Im just not sure what. To me it seemed like a family secret, something you dont tell other people. Nobody ever asked me about it...

That brings me to now. Ive still not spoken to anyone, my mum has moved on successfully with a lovely man and has two kids with him. But I saw that *** sister the other day and I just started shaking so I ran in the other direction. Mum doesnt know why I havent gotten over it but I cant forgive him for what he did to her and what he put me through...

Does anyone know what could happen if something like seeing this could do to a persons mind if left for so long? Thank you