Analysis and Suggestions
The problem here is thankfully sorta simple. You're trying to flip instead of trying to jump and then flip. Jump into the air, jump very fast and explode into the air! Don't try flipping until you're already in the air, until your feet have left the ground.
If you watch the video, you'll notice your legs never extend and push into the ground. They come off the ground, but that's because you're flipping back and pulling them off the ground. Your flip is very fast snappy and good! :) But work on jumping up.
Try delaying the tuck part of the trick just a littttle more, set the trick a litttle higher, practice getting up first, flipping second. You're on your way to a fast back flip! :D