Being self-critical is a waste of time. Try drawing instead.
Sometimes it seems impossible to begin painting. You may have no idea what to paint or where to begin. This may not be true for those that always paint the same subject in the same way, but for those that wish to create something unique, it can be a serious block.
I'm not going to pretend to know all the solutions, which tend to be as varied as art itself, I only want to present one easy way that has consistently worked for me in hopes that it may also work for you. Try it next time you are feeling uninspired and let me know how it turns out...
Grab a sketchbook and begin to draw without purpose, without aim, just feeling the pencil on the page and letting it flow. Drawing is easy at this point because it's not about beauty or skill and therefore it's not demanding. Don't waste a lot of time mentally toiling over what to draw, just jot down whatever pops into your head. You don't need direction. You will figure out how to pull something special out of it later. For now, just draw, discard, redraw, adjust, and draw again. It's relaxing and I guarantee you will get cough up in the moment.
This is called doodling and it is a useful way to get a kind of insight into your thoughts. For some people doodling may be absolutely crucial for creativity. Once you are "flowing" it will be easier to begin planning and to see new relationships and possibilities. Doodling is a motor act, and when occurring under conditions such as impatience, boredom, and indecision, it seems to alleviate those conditions. I'm sure there are complex explanations of how doodling reflects specific functions of the brain but let's leave that alone for now. All we care about at this point is satisfying our urge to create.
Note: The genius of artistic creativity, Pablo Picasso, essentially turned doodling into his practice. A sampling of his enormous body of work shows how he developed ideas right on the canvas using blind variations.