^^^ what CheoSamad said. It may be worth it to try a combination of experiments to see what works. Like any trick, try to work from the ground up. Like you said, you can grab on your 360s, so try and dissect what you're doing right there and advance it to your 540s. I'm in a similar boat as Cheo - I can float my 360s no problem but once I'm throwing 540s I definitely have to dig my edge in and wind up with my upper body to a certain extent.
As for your backflips, might be a bit tougher. Its always harder to just explain what you should do rather than to show you, but if you're over-rotating then really you need to not throw back as much when you're trying to grab, and maybe drop your legs earlier than you normally would when you're throwing a layout.
Don't know if you have one of these places near you but checkout SkyZone. They're a trampoline park company that have locations all over the US (I think). You can rent smaller foam boards to practice on too, and most of them have great crash pads and even a couple have foam pits. This place is basically how I learned to backflip well (amongst other things.) And no, I am not a SkyZone rep. My buddies and I just go there every Sunday night for an hour to practice and train. Good workout too. Good luck!