Is anyone else scared to be on top??

My SO always talks about me like I'm something special and someone anyone would kill to be with... But I'm 50lbs over weight, I have extremely limited experience and I have no idea what I'm doing... Like ever. He's always on top because I know that will get us both there but I fear he's going to get bored with me and feel like its routine. I know he would love it and its secretly what it hopes for I'm just scared that I'm going to ruin his fantasies. Has anyone else felt like I do and I have advise for how to overcome it?