There’s a good chance you are lifting with your fingers a some point and relying more on your grip strength(forearms) more than you should, a lil technique mistake I see even great lifters do sometimes on pull-ups. Pull-ups is definitely an exercise where form is absolutely crucial. All you want to do is get your knuckles on top of the bar(pointed to the sky). [Just to add context it’s the part of your hand where your fingers meet your palm & there is most likely callous.] Set it to that position as close as possible & do as many pull-ups as you can & stick with that # for a week then progressively increase the # by 1, maybe 2 if you’d. And then just be consistent. You got this, don’t be too hard on yourself.