WordNet sense 1 (requiring full use of your abilities or resources):

challenging

WordNet sense 2 (not easy; requiring great physical or mental effort to accomplish or comprehend or endure):

difficult, hard

WordNet sense 3 (strongly motivated to succeed):

determined

WordNet sense 4 (desiring or striving for recognition or advancement):

aspiring

WordNet sense 5 (marked by aggressive ambition and energy and initiative):

pushing, pushy, pushful