Add the below lines to the bottom of the skyrim.ini in the My Documents\My Games\Skyrim\ folder to fix the mouse horizontal sensitivity.

[Controls]
fMouseHeadingYScale=0.0400
fMouseHeadingXScale=0.0200

PS. These are my settings and I tried 0.0800 for y scale at first but it was way too much but 0.0400 is working well for me now.