Running out of memory despite having plenty?

So this is a problem that keeps happening to me. I have 16GB of RAM, and a minecraft RAM file that opens the program with 14GB able to be used by the program. Despite this, I can't enable SSAO, let alone shaders for the game. This should be more than enough memory to run AT LEAST 2x SSAO, but the client crashes with a memory error every time I even try to load it.

Help?