Complex sentence: Jennet went away for a year but after realising she could not be parted for so long from her son, made an agreement to stay at Eel Marsh House with her son, so long as she never revealed her true identity to him.
Split into simpler sentences: Jennet went away for a year, but, after realising, she could not be parted for long from her son. She made an agreement to stay at Eel Marsh House with her son, so long as she never revealed her true identity to him.