Accepting negative emotions and thoughts can reduce them.
The idea is a cornerstone in many ancient eastern philosophies, and is also backed by modern western science. This way of thinking has truly stood the test of time. The practice of accepting emotions have existed in Buddhism for thousands of years. Recent research has validated the benefits of this way of thinking, and modern psychotherapies include it, like ACT – acceptance and commitment therapy, DBT – Dialectical Behavior therapy, and is similar to acceptance as the last stage in the grieving process.
By acceptance, it does not mean simply allowing and being okay with negative things happening to you, or being mistreated by others.
But rather thinking and experiencing your negative thoughts in a non-judgemental way.
There’s several ways to look at acceptance. One way is dissociation. Which emphasizes that negative emotions are not part of your identity. They are not you, so don’t be heavily invested or feel intimate with them. They are simply thoughts that come and go. They are not nothing but electrical impulses in your brain and hormones in your body.
Another way is to see negative emotions as part of human nature. This is heavily drawn from Buddhism’s “Life is suffering”. Negative thoughts are a part of being human, and everybody feels down from time to time. That’s what makes us human.
The important part is to not try to run away from negative thoughts. Don’t feel bad, about feeling bad.
As famous Tibetan Buddhist Mingyur Rinpoche says: “Don’t fight with your emotions”.
Or as renowned Swiss psychiatrist Carl Jung says: “What you resist persists” – meaning that if you try too hard to resist negative thoughts, they tend to persist instead.
A mindfulness practice is all about accepting thoughts and emotions.
it bridges the get between spiritual and religions traditions, and science. Mindfulness practise exists in some form or another in all of the major world religions. At the same time it has also been rigorously studied and found to have positive effects.
In one study of over 1,000 people, they found that accepting mental experiences was related to less anxiety and depression and more life satisfaction. This was even when “controlling” for potentially related variables, like cognitive reappraisal (re-thinking something to make it more positive/less negative) and rumination. This means, basically, that the effect persisted even when those other variables were accounted for.
In Study 2, these researchers measured people’s general level of acceptance of their negative thoughts and emotions. They then exposed participants in a laboratory to a variety of stressors. Participants with a higher level of general acceptance experienced lower levels of negative mood as a response. 
Just as you shouldn’t try to run away from negative emotions, you also shouldn’t be overly concerned with happiness as a primary goal. As concentration camp survivor and founder of logotherapy Viktor Frankl writes: “Happiness can not be pursued: it must ensue. Happiness must happen, and the same holds for success: you have to let it happen by not caring about it.“
Again, accepting your mental states and your negative moods is not the same thing as accepting your life situations or allowing people to mistreat you.