Struggling to nail the rhotic 'r' vs the retroflex 'zh' (ழ) sound

I’ve been recording myself trying to master the 'zh' (ழ) sound in words like 'Tamil' and 'pazham', but my trainer keeps telling me I sound like I’m using a standard English 'r'. Does anyone have specific tongue-positioning tips or exercises to help me differentiate these retroflex sounds? I feel like I'm hitting a wall despite constant practice.