For some reason, if I ever have a dream where I have magical powers, they only work for a very short amount of time, then they just stop working and I become useless. Why?
Also, how do I figure out what my biggest fear is through dreaming? I have no idea.
This might be based on expectations. Because one time the magical powers worked for a short time, you imprinted in your subconcious that magic powers only work for a short time. In order to change this you must ‘replace’ the ‘magic powers work for a short time’ by ‘I can do whatever I want in dreams’.
There would be multiple ways to do this. You could try to ask for it in a lucid dream.
Scenario: You stand in front of a door and ask “Reveal my biggest fear behind this door.” After which you open the door and enter to see what that fear was. (You can be creative with trying to find out what your biggest fear is, remember it’s a dream so anything works: just expect it to work.)
Alright, thanks. 
I was thinking about asking for it in a LD, but I wasn’t sure if it’d work. Now I just need to get a LD.