Focus on the facts, because often the brain plays tricks on people and you get so lost in made-up scenarios that you take them as facts and feel bad or sad about it. Here it can help to refer to reality, so what really happened and what do you know for sure. Only these things count, because all the negative aspects you make up about it are just a figment of your imagination.
Speak out the thoughts, because problems and worries often become three times bigger in the mind than they really are. To find out how serious a fear really is, confide it in someone close to you, because they can mirror how bad it really is, i.e., a conversation is often the most effective way to resolve negative thoughts.
Condition the brain to focus on positive things. This method takes a little practice, but it helps in the long run. You should get into the habit of answering negative thoughts with positive ones. This may seem strange the first few times, but in the long run, such affirmations will creep into the subconscious and solidify there. As is well known, the power of thoughts works not only in the negative, but also in the positive.