Hi. What you are referring to is not psychotic. Many people unfortunately have various forms of anxiety. Naturally, there is no one simple answer that will entirely cure your anxiety.
Nonetheless, a general rule to remember is that we tend to either attempt to ignore our irrational fears or we try and remind ourselves that they are, in fact, irrational. These strategies might work for some of us for a short period of time, but they can actually increase the underlying anxiety. This is due to the fact that anxiety (as opposed to normal stress) is unconsciously-driven. Trying to counter an unconscious fear with rational reasoning is like trying to consciously change the path of a dream whilst dreaming. The more we ignore the specifics of the anxiety, the more unconscious it becomes. I would imagine that your fear hasn't been thought through beyond "There's someone under my bed" for a long time (if ever).
A more appropriate strategy would be based on focusing on and analyzing the fear. This would include following your fear through from the first hint of fear to the worst-case scenario, not leaving out any steps. For example: 1) someone is under my bed; 2) They will jump out; 3) They will grab me; 4) They will begin punching me; 5) They will force me to...
This can be done at neutral times and practiced untli you are able to follow this reasoning during actual moments of anxiety.
Because your fear is not clearly focused on, it is relegated to the realm of the unconscious. Since the fear is unconscious, the conscious mind cannot effect change in it until it becomes very clearly conscious.