Use the arrow keys to move your character and jump around. You have to get items that are spread through the level.
But beware, your past comes back to haunt you.
Windows: Unzip -> run ihatemyself.exe
OSX: Unzip -> run ihatemyself.app
Linux: Download and install Löve 0.7.2 at www.love2d.org run 'love ihatemyself.love'
Source is also on Github: https://github.com/joeka/I-hate-myself