Hi!
I am using the free hand path tool to smooth my lines, but it cannot find an option to make it sensible to the pen pressure to control the thickness.
I tried free hand brush with the stabilized, and it works very well to some extend, but it is not appropiated for short lines or closed curves.

Are you using a drawing tablet? Because regular computer mice don't do pressure sensitivity. Other than that, this page covers some of the common drawing tablet solutions: https://docs.krita.org/Drawing_Tablets

Yes. I am using a Surface Pro with a pen with pressure sensibility. I've already solved the initial problems with the drivers and I can draw lines and controlling the thickness with the Brush, but not with the Free Hand Path tool.

Oh, wait the freehand path tool, not the freehand brush? No, the freehand path tool doesn't detect pressure sensitivity at all. It's not designed for that.

Ops! Well, maybe we can move it into suggestions.
I feel much more confortable drawing with the free hand path. It provides smooth lines and you can also make slight corrections after been drawn, but it lacks that organic look.