You've learned about dealing with unwanted thoughts and emotions. Identify, ask why, and challenge with logic.
What do we do when there is no logical challenge to be had? Let's say we did or said something we regret. There's nothing that can be argued - we did it. There may be no opportunity to make things right, especially if it's something that's in the past. But we still feel bad. Or someone else said or did something to us that we're still carrying. Now what?
Emotions usually spring up from thoughts. We think about or are reminded of something that generates regret, anger, embarrassment, or another negative emotion. Maybe we're worried about something that may or may not happen in the future. Whatever it is, it isn't happening NOW and we can't control it. But we CAN control the thought that is happening in the present to keep it from spoiling our happiness.
Start as you normally would - IDENTIFY. "I'm feeling angry because..." "I'm feeling regret because..." Then tell yourself, "IT'S JUST A THOUGHT. THAT'S ALL IT IS." We're experiencing an emotion generated by a thought. It's just a thought. That's all it is.
Next, bring your attention back to the moment at hand. Observe any residual emotion and what is happening around us in the same way we observe what's happening when we're dreaming. In a dream state, we notice all kinds of little things that we might not normally see. View your environment in the same way. Let the past and the future stay where they are. It's just a thought. That's all it is.