What is the opposite of "to reminisce"?

ruminate

Rumination (psychology)

Rumination is the compulsively focused attention on the symptoms of one's distress, and on its possible causes and consequences, as opposed to its solutions. Rumination is similar to worry except rumination focuses on bad feelings and experiences from the past, whereas worry is concerned with potential bad events in the future. Both rumination and worry are associated with anxiety and other negative emotional states.
_{Wikipedia: Rumination}

Is Dwelling on the Negative Hurting You? The Cognitive Costs of Rumination

You are sitting at home and your mind keeps going over the negatives -- over and over. You keep reviewing what has happened to you -- perhaps a conflict with someone, something at work, your living conditions, your finances, your health. Whatever. You dwell on it. You are stuck.

What is Rumination?

A lot of people sit at home, dwell on the negative and find themselves getting deeper and deeper in their depression. Psychologists call this style of repetitive negative thoughts "rumination." When cows ruminate, they chew on their cud, chomping over and over without swallowing. When humans ruminate, they repeat negative thoughts over and over, dwelling on something either in the past or the present -- but do nothing to change anything. Ruminating is like spinning your wheels in the mud. You don't seem to be getting anywhere, so you just keep spinning your wheels, faster and faster. You keep digging a hole, find yourself stuck, and dig deeper and deeper.

Examples of rumination include repeating in your mind negative experiences in the past, replaying conversations that you had, dwelling on the "injuries" and "injustices" that you have suffered, or asking questions that don't have answers, such as "Why am I so depressed?," "Why me?," "What is the meaning of all of this?" or "Why did he or she say that?" You may ruminate about your physical maladies, your aches and pains, your emotions, your sensations or just about anything. The key thing is that you are stuck.
_{Huffington Post: Is Dwelling on the Negative Hurting You? by Robert Leahy, Ph.D.}

If you can't get something unpleasant out of your head, you would agonize over it.

agonize (verb) - to think or worry very much about something; to cause to suffer agony