Imagine if you had a radio commentator analyzing every move you made, everything you did or didn't do, in critical terms. "Jonathan is not having a very good day, no. Look, he's procrastinating again. Why can't he do better, I just don't understand it...." When you went out of the house, you would get more criticism from this radio narrator about yourself, your inadequacies and failings of various kinds, and it wouldn't stop all day long. You would probably want to turn the radio off, right?
A lot of people have that, though, in their own heads, and don't know how to turn it off. I, for one. Where is the switch?
Here are some ideas.
*Try to find that other narrative, the one that tells you you great. It sounds a bit corny and embarrassing.
*Avoid negative people.
*Figure out where that voice comes from. Parents? Ex-spouses? That voice is not really your "self" talking to you but an interiorization of other voices.
*Get into groove-like activities in which that voice isn't there. For example, when I am explaining my project to my friends, I just feel empowered, and the voice isn't there. Keep as busy as possible, but in good, supportive activities.
*Meditation is great, but it also gives space for that negative voice to be heard. If you aren't great at meditating (as I'm not) it might be because the idea of sitting there thinking about nothing allows for the worst thing in the world, which is called rumination. You still have to try it, but just be warned that that will happen at times. You can't be afraid of the voice, you just have to gently put it aside every time.