Useless. Quoting: Psych
One should throw away the negative thoughts of pessimism and lack of confidence in the success of the work at hand.
One of the most common mistakes made by self-help guru's is that that they don't know how to practise it themselves and therefor can't explain how to achieve that.
The key is to using forethought and pleasure (punishment) thinking BEFORE you make a choice of continuing the negative thoughts. It is not easy, especially in today's world. Negative thoughts bombard us from every which way, in a way it just makes us stronger.
It's okay to think a negative thought for a time, analyze it, figure out the *root* of WHY you think this is a negative thought...maybe the thought is trying to 'show' you something. You must slow down enough to figure out "why". If we're too busy (like most humans) we will respond to ANY AND ALL stimuli emotionally (mammalian brain) or worse, instinctively (reptilian brain). We all need to slow down and use our limbic brain and ask "WHY?"
For example, I struggle with procrastination, so once I notice I'm making excuses in my head for why I can't do something right away I stop the thoughts dead in their track and remind myself of the things I accomplished the day/week before. Once I start recalling the tasks I completed the day before I start to smile and remember how much I did for my son, neighbor, stranger, etc.
^^that is what it means to transmute an emotion at a basic level. You take the 'negative' aspect and turn it into a 'positive'.
Making lemonade out of lemons...
May I suggest a simple mental exercise for you? Next time you're being pessimistic think about something that makes you happy, something simple like the last time you kissed your lover, visualize it, hopefully it will make you smile and see if you don't feel better. Even better is if you connect an action to this feeling, so maybe you give someone near you a hug.
We must not repress our negative thinking, we must 'work through them' and figure out the why.
Hope this helps.