Hej Guys :)
Thank you for all the answers and the upcheer!! Especially Lawnz, for this detailed insight; this resonated with me ;)
My circles are getting rounder and rounder, i just have to practise, practise, practise. Overwriting my movementpattern regarding circles in my brain :) Actually i improved quite some.
Maybe i will try a flexible ruler, will see :)
The techniques with holding pen perpendicular and looking at a circle is my almost daily practise now. I split the circle in 4 quarters and try to get an individual feel for every quarter and combine them when drawing, tracing etc. :) Imagining round shapes helps too. As i said, i'm drawing every day now and i realised i can visualize shapes much better now. Currently i'm combining shapes like squares, spheres, cones etc. which is a pain in the ass for the moment, but im progressing. Sorry for off topic ;)
So keep your pencils sharpened and happy drawing,