You have the angles wrong. 0 is for the LEFT direction and 180 for the RIGHT direction by default it goes to the side. So 180 and -180 is the same direction. You can look at this. The sprite has a instance variable to hold its current direction versus using a button. I set the speed to 0 instead of disabling it.
Here is an image to better understand. The angles (**FIXED I had it reversed)
[attachment=1:29mjvdr7][/attachment:29mjvdr7]
**I just saw that you wanted a button but not the standard one. You can have a sprite and when there is a tap on it to execute the ACTIONS. Also fixed the file that it will only set the speed if it is zero.