I drew a quarter of my element and i want to generate the whole part by using 'pattern mirror'. The actual element is totally symmetrical (as you can see in the first image sketched by yellow curves) but in my final results (the second image) that ain't so and I have absolutely no clue why!
Change your settings to be "Pattern Bodies", instead of trying to pattern the features. Selecting the 'geometry pattern' option may also help, but it's not as robust as patterning the bodies, as then you can be 100% certain that your part is symmetrical. You would use feature/face patterns if you wanted to e.g. have a circular pattern of holes in an otherwise asymmetric part.
When you have created a 3D part then you don't have to go to sketch circular pattern for an easy approach. Instead, you can directly go to feature toolbar there you will find and a ‘Linear Pattern’ below which there will be an arrow for a drop-down menu from there select 'Circular Pattern'. Or you can go to Insert>Pattern/Mirror> Circular Pattern and then you can perform the same step by selecting features you want for the circular pattern.
If you are doing it from ‘Sketch’ then you have to go to ‘Circular Sketch Pattern’ from the drop down of Linear Sketch Pattern. Then select sketches of features, not the features and then you can create a circular sketch pattern. And thereafter extruding the circular sketch pattern will give you complete part.