Struggling with 'O' vs 'A' vowel sounds in register consonants

I’ve been recording myself reading Khmer texts daily, but my progress on the inherent vowel sound change based on whether a consonant is 'a-series' or 'o-series' is frustrating. I feel like I'm constantly overthinking the registers mid-sentence and it ruins my flow. Does anyone have a specific drill or mnemonic they used to memorize the register of each character so it becomes subconscious instead of a conscious math problem?