as merged

[bosun]

Can you please explain what the following sentence means?( especially the red parts)

But many of our memories aren't so much lost or thrown away , as merged. As time passes, the details drop awa and only the broad outline remain.

[kingkoti]

That means our memory is not bad that we tend to forget quickly. we remember everything but as time passes those memories fade and only a rough idea of that instance remains.