I do a breathing exercise as first aid that helps me deal with anxiety and depression, but could work well as a first aid in a reality check.

Here it is in all its simplicity. But it works. The mind cannot focus on two things at once so when you count the breaths, there is little room for worry and a natural result is to experience relaxation. Great at bedtime, waking up or anytime I have a brain train derailment.

Silently count as I breathe naturally one on the inhale, two on the exhale, three on the inhale, continuing up to 10. Then start at one again.

For more info on coping with derealization.
Psych Central - Search results for Dealing with derealization