feeling uncomfortable that boyfriend is going to the gym

my bf never went to the gym but he’s planning on starting soon. i can’t quite figure out why that makes me uncomfortable but i think it’s because of my own stupid insecurities. i’ve always been insecure of my body, particularly my small breasts. i do trust him but i can’t help but think that he will end up looking at other women in the gym. they’ll be wearing their tight leggings with crop tops or sports bra and men will definitely look. i’ve told him about this and he promises that he doesn’t even look at other women in that way anymore but i’m so insecure that i always have a little voice inside my head saying that he’s lying. i’m so tired of living like this, of constantly being aware of the things that i hate about my body. it’s so draining honestly.