fiksacja Englisch:

fixated /fɪkˈseɪtɪd; US / adj. [not before a noun] fixated (on sb/sth) always thinking and talking about sb/sth in a way that is not normal He is fixated on things that remind him of his childhood. ▶ mający obsesję (na punkcie kogoś/czegoś)

My sister used to be really fixated on losing weight.