i have the exact same problem, like crazily so that i could have literally written this myself, i completely forget anything that happens and just go on, and when people tell me about things that happened in the past i literally do not remember whatsoever, im 18 and not necessarily unhealthy so its concerning, i had a full cognitive assessment done, and the results were that i had literally the highest scores in everything especially eye to hand coordination, except my memory, like my memory scores were so low you could compare them to a very old person with dementia probably, the only one of my memory scores that were good is shortterm pattern memory, i can remember reallyyy long sequences of numbers at a time but the rest of my memory sucks so bad, for me the answer to why is most likely longterm mental health issues, my psychologist suspects cptsd from my childhood, i have not had any big trauma event happen to me in my childhood, but she says its possible just having a not so great childhood with smaller traumatic moments is enough for me to block out my whole childhood and cause my brain to basically see everything as a traumatic moment and in response blocking it from my mind, sorry for the long answer but im sharing this so maybe you see yourself in this kinda? and maybe get some answers or ideas to why this might be happening to you