Emma did consider that option but she knew that, if she confessed at that moment then "mc would leave absolutely everything and everyone to be with her", (that was the exact monologue). It's not everything about hera. What about aine? As she was there when Emma needed her the most, what about her?
Even if she could have told him everything but then what, keep your relationship hidden, the relationship he absolutely cherished and even still do, that would be more cruel. Why not wait and go bazooka.