Most Helpful Guy
I have no clue whether this is mild or serious, but I would urge you to get some professional help. If there are times that "a different personality" takes over and you "kinda blackout", I really think you should seek help from someone who has lots & lots of training/credentials.
Most Helpful Girl
Dissociative Personality Disorder, formerly termed Multiple Personality Disorder.
A separate "you" has emerged to deal with anyone or anything that could trigger memories of a past traumatic event. This is your brain trying to protect itself. You should not feel ashamed, but the dangers of harm are very real, and you should seek professional help to overcome this.
Remember, it's not weakens to ask for help, it's taking charge of yourself and your own life. This is an affliction that was dealt to you, placed on you by the abusive actions of another, it's not your fault, but it is your cross to bare, so it is your responsibility to find help dealing with it. Good luck! You can do this!