How did Eren lose his memories? We all know by now that Eren


   has the Coordinate power, which means that he should possess memories of everything (origin of titans and such). 


How does he not remember everything until much later on?
Historically, the memory inheritance power of Coordinate would only work for the bloodline of the Reiss royal family.


  Nevertheless, it seems that only the Reiss bloodline can activate this ability from the beginning, since Uri and Frieda Reiss showed signs of being aware of that knowledge immediately after obtaining the Coordinate, while Eren was only able to see some vague memories of Frieda after years of having received it. 


One of the characteristics of the Coordinate is that its effects are linked to bloodlines.

The memory manipulation didn't work on several bloodlines that were not among the common people in the walls, 


  (...) it is known that at least the Ackerman bloodline, the Asian clan, and the noble families are immune to this mental manipulation due to not sharing a common bloodline with the majority race within the walls.


So by not being a Reiss by birth, the coordinate is more or less dormant on Eren.

Source: http://attackontitan.wikia.com/wiki/Coordinate