Changing myself and insecurity

My boyfriend told me he wishes i had a bigger butt and gained more weight.😓 ive been akinny all my life and am tryinf to wrokout and eat more but im just naturally skinny. This has been an insecurity most of my life and i feel not good enough and compare myslef to other girls thinking maybe he would like be better if i looked like them.. He is fit and loves the gym but says these things to me and it hurts even though i laugh it off each time