Why does my 'khai' sound like 'kai' to my Thai partner?

I've been recording myself daily to nail the aspiration differences in Thai consonants like ข vs ก, but no matter how much I practice, my spouse says I sound like I'm not making distinction between the sounds. I'm getting frustrated because I can hear the difference when a native speaks, but my tongue just won't do it. Should I focus more on the airflow or the pitch, and does anyone else have a specific drill for these aspirated vs. unaspirated pairs?