Sentence:  Failing to get rid of the old love [<nowiki/>Medea] before taking on the new—in other words, wasting his strength over a new and untried method before having fully established the old—he [<nowiki/>Jason] <WSD>fell between two stools</WSD>. 
Meaning of the word marked with <WSD> tags:  To attempt two roles and fail at both, when either could have been accomplished singly.