I assume your position for G is pinky on the e, ring on the B strings at the third fret, middle on the E at the third and index on the A string at the second.

Try focusing on getting the pinky & ring fingers down correctly and let the index & middle come down a nano-second after. In fact, you can practice with some improvising, getting the R&P down, strumming the lower strings, then bring the I&M down on the next beat and hitting them for emphasis or doing a full strum, whatever you want. (I hope that makes sense.) You can get a rythmn going with that, and be effectively breaking the G chord into two steps just by how you play it like that.

Although it seems you can switch from other chords to G easier, keep doing those changes as well. It's all about finger memory.