Baby fights every sleep

My boy is about to be 6 months old and for the longest time, he has fought and still fights at every nap time and bed time. I do my best to watch for wake windows/tired cues, and no matter when I go to put him down, it’s always the same. He will be happy and joyful when I pick him up to get ready for a nap, and as SOON as his body hits the crib for me to put his sleep sack on, it’s like all hell breaks loose. He knows that his crib means sleep and that putting him in his sleep sack means he’s going to be going down soon, but why does it have to be such a battle??? Some days I can soothe him pretty fast and he will suck his thumb and honestly go down quickly, but others days….my goodness, my child just screams and cries and it breaks my heart and I get frustrated at him because it seems like I’m doing something wrong when I’m just trying to get him to take his nap. Then I find myself annoyed and upset with him for fighting so hard, and then I find myself being upset with myself for getting frustrated with him and I end up feeling like an awful mother. I know I need to work on more patience and I’m trying with that, but please, is there something I’m missing here? Does he just have an awful case of FOMO? I feel like I’m the bad guy every time with how hard he fights. It’s like the end of the world when nap time comes. :(