Need help with back walkover pleez?

Okay, I really wanna do cheerleading in high school, so I've been practicing gymnastics daily all the time. I've mastered a cartwheel, roundoff, front walkover, front limber, and back bend. I can go back, i go up on my toes, I bring my feet to my hands, I just can't kick over! Before my coach said i should do back exorcises, but after like 2 weeks he said my back was fine, but I needed to work on getting a stronger stomach. Like how do i do that? Any good tips for back walkover. It's killing me! I wanna master this so i can focus on flips and back/front handsprings. Also, I get reall bad headaches from practicing these. suggestions? Should I take like advil before practice? Whenever I try to bring my hands closer I wind up just like HAVING to stand up. PLEASE HELP GYMNASTS!