Why wont mirroring of the Brush work properly? I want to do the texturing but I countered a problem I couldn't solve.

I thought the x-axis is red and the y-axis green.



But when I turn the mirror in texture paint on x-axis it mirrors the paint around the y-axis and with y-axis enabled the other way round.

On top with y-axis enabled it is strangely shifted.

I already tried setting the origin of the model to volume and moved the object to the center with Alt + G , but it didn't change. Also I tried to texture paint a cube in a new file to see if it's working there and it does properly. Next I tried to copy the fly to a new file and texture paint it over there but I encountered the same problem.
You may need to go in edit mode and select a single loop of vertex to snap the cursor to that will allow you to place the origin center so that your object will paint correctly. A common problem for mesh that has been moved in edit mode away from the origin is this effect, that the center will be offset incorrectly.