Rationality starts from evidence, and then crunches forward through belief updates, in order to output a probable conclusion. "Rationalization" starts from a conclusion, and then works backward to arrive at arguments apparently favoring that conclusion. Rationalization argues for a side already selected; rationality tries to choose between sides.
Rationalization can be conscious or unconscious. It can take on a blatant, conscious form, in which you are aware that you want a particular side to be correct and you deliberately compose arguments for only that side, looking over the evidence and consciously filtering which facts will be presented. Or it can occur at perceptual speeds, without conscious intent or conscious awareness.
Defeating rationalization - or even discovering rationalizations - is a lifelong battle for the aspiring rationalist.