How can I stop dreaming about my abusive ex?

Months and months ago I finally got out of a very abusive relationship. We lived together for about a year, and for the last 6–8 months of our coinciding he cheated on me multiple times and began physically abusing me. He would become angry at the most ridiculous things, and would show his anger towards me by dragging me down the hall by my wrists, pulling my hair from my head, choking me, and the last straw—hitting me. I am now in a very happy relationship with a man just like my dad. He is by far the best thing that has ever happened to me, and we are getting married in November. When I go to sleep with him, I feel safe, protected, and loved. The best thing is he is my brother’s best friend and my brother and myself know that he would NEVER treat me like that. However, I still have nightmares almost every night that I am still with my ex and he is abusing me. It is making me feel tired every morning when I wake up. Why am I having these dreams, and how can I stop them? Please help me!