I am aware of negative thoughts entering my mind

Sit in a stationary train while the one alongside pulls out and for a second you'd swear you were the one moving. Then the mind catches up and the whole sensation reverses.

Noticing a thought does something similar. Inside it, the thought isn't a thought at all — it's simply how things obviously are. The instant you clock it as arriving, you've moved to the platform side of the glass.

That's most of the skill, and it's much less than it sounds.

You don't have to disagree with the thing. You don't have to feel better. Registering that something has entered the room is already a different act from living inside it.