Consider the things you do remember, the things you know you'll never forget:
Why do you recall those significant things so clearly?
Probably because you were interested in them, they caught your attention, and gave pleasure. Attitude and attention to learning are critical for remembering over a long period.
If you're studying a subject that holds little interest for you, it will be harder to remember the content. However, if you're determined to pass because you want to be employed in a certain profession and this is one of the requirements to get there, your motivation may provide a sufficiently positive attitude for you to process and retain the information.
You need to notice what's important and focus on it to retain it. We hear, see or experience something through our senses, but if it is not reviewed within minutes or hours, it will fade. Repetition and rehearsal, however, will keep the information 'alive' in the working memory.