Complex sentence: It is possible Mary began her education abroad and spent time as a companion to Archduchess Margaret of Austria, Regent of the Netherlands, but it is believed that it was Anne who was chosen to go to the court of the Archduchess.
Split into simpler sentences: It is possible Mary began her education abroad and spent time as a companion to Archduchess Margaret of Austria, Regent of the Netherlands. However it is believed that it was Anne who was chosen to go to the court of the Archduchess.