If you're struggling with negative thoughts, achieving flow is probably the best medicine. Contrary to popular wisdom, forced positive thinking often makes things worse.
LessWrong • How to Be Happy - LessWrong
When people suppressed their fearful thoughts, as opposed to vividly imagining them, they were markedly less anxious about those thoughts later. They also rated their fears as less vivid and remembered them less.
Shayla Love • Should You Confront Your Worries or Try to Banish Them?
The attempt to suppress unwanted thoughts causes them to emerge into conscious awareness more strongly and more frequently than if they were given attention in the first place.
Kristin Neff • Self-Compassion: The Proven Power of Being Kind to Yourself
research on the subject of thought suppression2 clearly shows that the main effect of thought suppression is a resurgence of the thoughts
David Carbonell • The Worry Trick: How Your Brain Tricks You into Expecting the Worst and What You Can Do About It

Research has shown that minds are difficult to control (8, 22), however, and it may be particularly hard to steer our thoughts in pleasant directions and keep them there. This may be why many people seek to gain better control of their thoughts with meditation and other techniques, with clear benefits (23–27). Without such training, people prefer d... See more
Just think: The challenges of the disengaged mind | Science
In a study published by the journal Nature in July, a team of researchers from the University of Texas at Austin, Stanford University, University of Rochester, and Google Empathy Lab found a way to do that. The researchers used a 30-minute online training session to teach participants about a mindset that sees stress as an opportunity to learn and ... See more