Recurring, horrifying nightmares of my ex...WTF?!

It's been almost a year? I FINALLY started feeling a bit better. ALLOWED myself to be just a little happy. Now I have constant nightmares. Some are run of the mill fights, or scary situations. The worst ones are when we are happy together. Fond memories, moving forward with our relationship, marriage, making a family.... I wake up in a cold sweat and cry through the night. WHY? This is stupid.