Complex sentence: He first entered the professional tour for the 2004/05 season but was unable to retain his place for the following season's tour, but regained his place for 2006/2007 by winning the European Under-19 Championship.
Split into simpler sentences: He first entered the professional tour for the 2004/2005 season, but was unable to retain his place for the following season's tour. He regained his place for 2006/2007 by winning the European Under-19 Championship.