Attitude Towards Speculation and Fantasy I'm experiencing a lot of speculative thoughts and fantasizing about the future when I'm going about my day. I'm wondering whether I should attempt to deny these thoughts, or merely let them arise without judgment. I'm also wondering whether engaging in concentrative breathing meditation will reduce this thoughts in the rest of the day, and whether the arising of these elements or other negative elements such as anger, lust, hatred, pride and narcissism can impede spiritual progress. 

I think the main question is: should one allow such thoughts non-judgmentally, even if they are delusional, or should one reduce them e.g. via the breath. The speculative and fantasy related thoughts seem to take my time, and so I wonder how one can let these thoughts arise in non-judgment without detrimental effects.
There are two methods to reduce thoughts: 

(1) Wisdom method, which is replacing unwise thoughts with wise thoughts. This will reduce the thoughts &amp; build the wisdom that stops the re-arising of such thoughts in the future. Examples of wise thoughts are exactly what you have posted, which is thinking to yourself: "These fantasies are delusional &amp; are a waste of time". 

(2) Concentration method, which is to use the breath to feel calmness. When the mind feels calmness, it will not be inclined to think unnecessary thoughts. 

Before the Buddha attained enlightenment, he generally used the wisdom method to reduce his unbeneficial thoughts (refer to MN 19). This is called 'yoniso manasikara' or 'wise reflection'. 

The mind is difficult to know clearly &amp; manage therefore this is a task we each much learn. The Dhammapada states:


  33. Just as a fletcher straightens an arrow shaft, even so the discerning man straightens his mind — so fickle and unsteady, so
  difficult to guard.
  
  36. Let the discerning man guard the mind, so difficult to detect and extremely subtle, seizing whatever it desires. A guarded mind brings
  happiness.


In your situation, the wisdom method is probably the best method since you ask lots of questions here about meditation, which shows you probably need to know your mind more. 

As for 'non-judging', the Pali suttas do not really teach the practise of 'non-judging' towards unbeneficial thoughts. In addition, fantasies themselves are 'judgmments'. Therefore, if the mind was really abiding in 'non-judgment', it would not be fantasizing about the future.