To trigger the next memory, find De La Serre (exclamation point on map) and interact with him. A small cutscene will start with your next objective, to deliver a letter to him in the moving carriage. It doesn’t matter how fast you go, you won’t be able to reach the carriage, just keep up with it by cutting off the route over the rooftops and you’ll eventually catch up to to it, though it’s the wrong one.