Drifting off is totally normal, and it sounds like you’ve been able to eventually catch yourself as it happens. One thing a lot of people find helpful is creating a cycle of positive reinforcement, and turning that point of realization into an “aha!” moment, where you say “oh good, I noticed I drifted from the meditation object, good job me :)”, even putting on a little smile for yourself even if it feels weird at first. Soon you find yourself having these Aha moments automatically whenever you drift, until eventually you stop drifting completely.