As a good starting point, you can counter the next negative thought you have about yourself with a positive one. For example, "I never do anything right" you can counter with, "I'm doing the best I can, even if it's not perfect". Instead of calling yourself "worthless", call yourself "worthy". When you're dealing with changing habitual thoughts, it's best to write things down so that they don't get lost in the shuffle. Write down positive thoughts about yourself. Write them down every day, and read them multiple times to get the new habit into your head.

One night I had bad insomnia, a lot of stress, and negative thoughts about myself. Finally, after some hours of misery, I decided to just write something very simple- far from effusive praise. "I am okay. I am an okay person."

I wasn't about to write that I was great. I was just okay... and that took a huge weight off my shoulders. I was able to go to sleep right after that.

So, in a nutshell, affirmations. Listen to positive affirmations, read them, write them, say them out loud to yourself, and think them. You don't have to try too hard to fill your mind with ONLY positive thoughts so they end up causing you stress, but ease into them, and writing them helps so you don't have to think too hard.

That being said, whenever a negative thought comes up, do take the time to STOP the thought in its tracks, and say that this is an illogical thought. It has no real basis, and there's nothing meaningful or productive about this thought. I choose to think something better. The positive thoughts are more real than the negative thoughts.