Hello. My ex broke up with me and I am trying to bring her back using magic. Since my last spell I had these very bad dreams about her leaving me and not being able to get her back, even if I am very positive about my spells and I simply know she will come back eventually. The last dream I had about her was today. I dreamt that she was flirting with someone new and I killed that person. I could clearly see that girl’s face. She was wearing red and her name was Nina. In my dream she was like a spirit. Somehow she was very similar to me but she was evil. She was very strong and almost lost that fight, but I was stronger and killed her. Then, I killed my ex also. Now, don’t get me wrong, I would not kill anybody and this scared me. The fact is that I was the powerful in this last dream. I was one deciding who lives and who dies. What do you think would be the meaning of these dreams? I should mention that my ex is not into someone new and she is not a witch. She is pretty healthy and wouldn’t get anyone just to get over me.
There does exist a demoness by the name of Nina. I have no more information to provide.
It sounds like this dream is confronting you with your own jealousy, and possessiveness over your ex.
Likely this about your unconscious mind.
You probably have some internal work you need Todo about this situation and these dreams are your mind trying to get you to do that.
I can’t put it better than @crookedpathfinder on this one. It sounds fear based.
I’ve had a similar thing in reverse just yesterday, a potential FWB accused me of sleeping with someone else because he had a dream about it and he thinks his dreams are always real… we know they are very much symbolic and not ‘real’ in the sense of being something you can take at face value in the physical more than very, very rarely. Of course, I haven’t, as it happens, and it’s not his business either, so I’m reading it as a worry dream. Kinda cute, really. 
Was also thinking it could be a message from the unconscious. Maybe Nina is your bad side which you have to face and integrate to get your ex back?
Maybe having control over someone else (via magick) deep down scares you (was thinking that because the killing scared you)?
Ultimately you have to sense what rings true and what doesnt, Im only throwing out guesses here ofc.
What really scares me is that entity. She was the only one who seemed to be real. I guess she is the part of me that I try to kill or supress, but in the end I guess she won because I killed my ex in that dream.
Maybe it was your subconscious or unconscious mind preparing you in the event you and your ex don’t get back together. I know your conscious mind is confident you will, but you don’t know that for certain.
This dream could be the rest of your mind pre-conditioning you so in the event you don’t get back together, you don’t get hit with this info like a ton of bricks.
I see the killing as a metaphor. You’re not literally killing her but you’re telling yourself you will have to “kill” her out of your life if you don’t get back together.
Nothing has meaning unless you give meaning to it.