Wow, how terrible for you to have experienced such a horrid person.

My thoughts on why this is haunting you is because you are keeping it too yourself in away. While I would imagine its a deeply personal & would be hard to talk about.. Not telling DP about the details.. I dunno. Your worried about what he might do to the ex.. Why? Your DP is protective of you, I think would mean he would also be very supportive of you. Let him into that part of your life, emotional well being.

I have never been in your shoes, but I think letting your DP know its troubling you & that you would like his support. When you are ready to speak about it with him, you would like him to be supportive of you in the Now. Maybe speaking with a counsellor who can then be with you while you explain the deeper details of the abuse to your DP. Someone who can help him understand how hard it is & why you had not told him before..

It must be very difficult. I really think if you open up with your DP, you will find some inner peace.