girl on top tips please!

last night my hubby said we aren't having sex until I'm on top. I'm afraid to do cowgirl or any positions with me on top because I'm bigger and I'm afraid that it'll turn him off or that I'll hurt him. because the first time I tried cowgirl the guy went flat and my confidence about it dropped super fast. ive watched porn but the majority of those girls are super thin so it doesn't really help. So please give me some tips on how to be confident on top and how to make it work.