I know the more positive thoughts I have the less negativity can creep in

Gardeners who hate weeding plant thickly. Not because ground cover kills anything, but because a bare patch of soil is an invitation and a full one isn't. The strategy is occupancy rather than combat.

Attention behaves a little like soil. A mind with nothing much in it at four in the afternoon will grow whatever seed is lying around, and the seeds lying around are rarely the flattering ones. That's an argument for keeping something absorbing to hand — a difficult recipe, a book you're genuinely gripped by — rather than for trying harder to be cheerful.