Always be aware of what you're thinking. Guard your thoughts. You are not your thoughts.
You are a witness of your thoughts. Observe your thoughts. Are they serving you? If not drop them.
Do not allow yourself to become emotionally attached to negative thoughts.
This can be a vicious circle if left unchecked.
The negative thoughts, if persistent generate negatively charged emotions and these in turn strengthen the negative thoughts causing more emotional turmoil.
Choose to have positive thoughts over negative ones and take back control.
A lot of people have got into the habit of negative thinking and don't realise that they have the power to change it. It just takes a bit of conscious effort to notice your thoughts and analyse them. Your mind can be compared to a computer, a computer with stored programs. We are taught how to act and behave from an early age, inheriting "programs" that have been passed down from generation to generation. Analyse these programs. Are they still valid today. Drop the program or rewrite it to suit you best.