Complex sentence: When people cling rigidly to their mental sets, they are said to be experiencing fixation, which is thus the psychological term used to describe a seeming obsession or preoccupation with attempted strategies that proved unsuccessful.
Split into simpler sentences: When people cling rigidly to their mental sets, they are said to be experiencing fixation. Thus fixation is the psychological term used to describe a seeming obsession or preoccupation with attempted strategies that proved unsuccessful.