It simply does not pay to put yourself down when you are on a path of self development or self improvement. For every little success you have will be negated by the negative self talk or attitudes you have towards yourself. It's like one step forward, negative self talk, then two steps back.
Wherever you want to get to in life, whatever you want to do, it is imperative that you become your own best friend first. Why bother with self doubt and loathing? Look where they have got you? Put them behind you, they haven't served any useful purpose up until now and they won't serve a useful purpose in the future. All they do is keep you stuck in a self perpetuating loop of negativity and self doubt.
In what situations do you speak negatively towards yourself? Maybe you make a mistake at work and say to yourself 'I'm an idiot, I always do this'? Or maybe you left the house one day and forgot to bring some important paperwork with you? Did you remind yourself that you are 'useless person who always forgets things'? Or perhaps you said something even more severe?
It's not useful and it gets you nowhere. Instead it just leaves you feeling negative and bad about yourself.
In this day and age, time waits for no one; to be successful you need to be on your own side. You need to continually support yourself through everything you do, it is important to give yourself praise when you make progress (however small) and not to berate yourself when things go wrong.
Life can be hard enough without you being your own worst enemy. Always be your own best friend. Learn to accept yourself for who you are at any moment in time.