Here is the moment from a story:
For the first time after several years, he laced up his shoes and stepped outside to start his run. However, the run turned to a walk as his thinking turned inward when an overwhelmingly intrusive and unwelcomed thought sapped his motivation towards sorrow.
Is there a word that captures the emotional shift of the runner? Some options that I have considered are whiplash, instantaneous change, discomposure, turmoil... However most of the words capture a recurrence or tension between two ends and a related flux. I am struggling to find that one word that describes this moment when emotion stifles the will to continue more energetic movement.