If you are unable to fall asleep during WILD attempts you are simply trying to hard. It's hard to describe but WILD's are best done passively. If at any point during your attempt you think about giving up the attempt, you're thinking to hard! WILD's don't require that much active participation. After a WBTB set your intention to have a WILD, then just let go. If you fall asleep no big deal. Next time just let go more slowly. It's always better to fall asleep in the process than stay awake.

For a little perspective I attempt a WILD at least two, sometimes three or more times a night. I've made close to 1000 attempts. Around 50 of those have been successes.

Learning to WILD is a super delicate balancing act. Failing in your attempts is not a big deal at all because every one will help you move closer to finding the balance. And once you find the balance it will be easier to find again. So be patient and enjoy the game.