I was told that once your memory capacity is filled, new memories overwrite the older ones from oldest to newest.
I think that it would make sense if we had more control on what to overwrite. Not only it is handy but also more realistic. For example if I get a new phone number IRL I gradually tend to forget my old number and I start to more easily recall the new number. I won't forget a totally unrelated memory because the new number is in its place.
It's not particularly important but still I think it'd be better if it worked that way.