Why does my tone in 'shǐ yòng' sound like 'shì yòng'? Heritage learner here.

I grew up hearing my parents speak Mandarin, but I never mastered the tones. When I try to say 'utilize' (shǐyòng), it sounds exactly like 'try to/attempt' (shìyòng) to my mom. How can I practice distinguishing these third and fourth tones in natural conversation without sounding like a robot?