OK, first, do what spiritalk advices: eliminate all negativity in your life, all sources of fear. I know that I myself lived with great fear until I changed my religion and habits. Since I'm not playing fantasy games and don't watch horror movies, I feel no fear. Your imagination may be hurting your morale and causing the impression that  "demons want you dead".
 Secondly, I know from my experience that demons are belevolent beings who love humans and would never hurt them. Christian angels are painted after depictions of demons.  
 Third: even if demons were as you believe they are(evil), they would have no reason to send an assassin after you. If you survive a spiritual attack from a devil, the attack was not meant to kill you. Also, do NOT associate devil with demon. Everyone knows what devil means, but very few know that A Demon is a messenger of the Ancients (Original Gods) to humans - this is the supposed role of angels in christian mythology.
 Fourth: Do not worry about this too much, because your imagination makes your assassin real. Some people are born with more spiritual power than others, and are unaware of it. You may be able to create such vile entity, a servitor, and you just suggested it that it should attack you. If it appears again, try to use autosuggestion and give it messages in your head, try noticing it's behaviour.
 Fifth: to answer your question, meditate on your fears. Eliminate them all one by one, give them orders to leave. Do this on a daily basis and you will notice results.

I really hope this helps you solve your problem.

Always meditate on a scene from nature.  Nature offers us so much to emulate - we are nature after all, human nature.

Try The lesser banishing ritual of the pentegram. Just reading through the how too of this you will answer 99% of your questions, this is a very powerful banishing ritual. most that use it do it before meditation,rituals, anything. try it untill you can do it with out looking at paper and let me know how it works for you.

Sounds like you've allied yourself with the wrong type of demon, weirdoo. There's a war going on in this world and only one side is winning. I suggest breaking off contact with the side going against Lucifer. They can't win against him and Lucifer isn't taking any prisoners.