Feeling guilty about things that have happened in the past or thinking that something horrible is just around the corner is central to most anxious and depressing thoughts. To keep yourself in the here and now use these techniques:
Sit quietly and take note of your surroundings. Notice the light, sounds, and everything around you.
Avoid the temptation to judge or evaluate, and just observe.
Notice your breath going in and out of your body.
Notice how your body feels as you sit.
Sit, breathe, observe, and be in the present. If your thoughts start to dwell on worries or concerns, just pull yourself back to the present.