As a Halo fan, I feel your pain, and it isn't news we want to hear. But it's true, you're not going to be seeing any previews of Halo 6 at E3 this year.
A representative from 343 Studio hit up fans on Reddit this week to address the rumors that we'd at least see a trailer during the Microsoft presentation. 343's community manager Brian Jarrard (Ske7ch) stated that "we'll have a little something at E3 but it's not related to the next major entry in the franchise." If the timeline holds correct for the primary Halo games (the ones that have numbers), you're not going to see a sequel for at least three years. Since it hasn't even been two years since Halo 5 was released (October 2015), we got at least another 18 months of waiting to do. Just keep calm and know it's probably going to be in 2018 when you get a proper game for the second trilogy.