Dreams can be pretty crazy things and the depths of your mind can churn out some bizarre stuff while you are experiencing them.
It sounds to me like a particularly intense dream was triggered by this stressful period of your life. I myself have experienced the sensation of a lingering 'dark presence' following dreams many times. I personally attribute this to the fact that you can go into intense and sometimes odd mind spaces when you are dreaming. If you awake abruptly then you are still essentially in the same emotional state and it takes a few minutes for you to 'normalise'. Hence you will experience heightened terror, if that's what you were experiencing during your dream.
Kato R Deprete:
When I dont think about that experience or anything negative I sleep just fine but some nights I cant help but feel like a darkness is there or its just my mind toying with me
I believe you've answered your own question here, i.e. 'when I don't think about...' Your thinking about it creates a negative reinforcement loop of fear. It's just in your mind! There's nothing there! Even if there was something there, can it do anything to you? I remember as a child seeing something on TV where a monster's hand came out from under the bed and grabbed someone's ankle. Following that I was often afraid to get into/out of bed at night, usually opting to take a running jump onto the bed, thus ensuring my safety. Once the idea was there it was difficult to shake... Just remember, there's nothing to fear but fear itself! You don't even exist, apparently...
Kato R Deprete:
I would like to know why the hell that had happened to me when I was young and vulnerable,
Because you were young and vulnerable and going through a traumatic period.
Kato R Deprete:
what the fuck was it and why I haven't had it happen again?
See above, and because you are not as young/vulnerable and have not been through a trauma like that since. Also there's no guarantee that a traumatic experience will yield a bizarre and terrifying dream.
Kato R Deprete:
What can I do to protect myself?
What do you need to protect yourself from? Fearful dreams? Dark spirits? Fear itself? Suffering?
Chances are that you will occasionally have fearful dreams, perhaps not as intense as that one though.
I don't know about dark spirits/forces, but all you can do is observe your own senses and thoughts. Don't worry about anything else.
Fear is a form of suffering that results from attachment/aversion (fear of losing something/or of something occurring) to phenomena.
The goal of meditation is to free yourself from all suffering by observing the 3 characteristics of everything you perceive.
So I guess you should meditate!
That said, if you want to "grab your balls and go for it" then you will most certainly have some intense dreams on retreat and no doubt encounter your fears along the way (see dark night).
That's just my humble opinion anyway; hopefully you'll find some use in it.