Struggling with the 'ije' vs 'je' distinction—is there a rule to stop guessing?

I’ve been studying Croatian for six months and I still constantly trip up on ijekavian spelling patterns. Every time I write, I have to look up whether it’s 'mlijeko' or 'mleko' in my head, and it’s killing my flow. My instructor told me to just memorize the base form, but is there a reliable phonological rule or trick to master these vowel changes before I start confusing the hell out of native speakers?