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.