can i fantasize

Do not Fantasize. Whenever I fantasized , I became weak and a relapse surely followed after a week or so.
I realised it quite late that fantasizing was the reason behind my previous relapses.
And yes, it also take a tiny bit of effort to stop your brain from fantasizing because fantasies come randomly and if you don't stop at that very moment, then they keep on going.

 

Thinking about becoming a superhero and saving the entire universe and all that is called Daydreaming, which if done in limits is absolutely normal. If daydreaming is done too frequently and it affects your life in a negative way, then it is called maladaptive daydreaming.

Sexual fantasize is not good and can be considered same as PMO.

 

Limited Daydreaming wont while, sexual fantasies will.

 

I also fantasize in a similar way. (crush, being a hero) At one point I started quitting it altogether and it changed my life.
Try to give it a break.