Rather than trying to suppress the thing you don’t want, try to nurture the thing you do want. I suspect (but don’t know) that suppressing will have weird side-effects and not actually provide much growth, but if you nurture the thing you do want then the unwanted characteristics will naturally atrophy.

In your case, try to nurture positivity instead of suppressing negativity. Perhaps write a blog post on something you are excited about. Or write more comments of the “Yes, and…” variety.