im not sure if this will make sense, but my trainer explained it really well to me when working on leg. grab your horses mane, stand up in your stirrups (half seat), and lean back until you can sit up and your leg can support you. maybe youtube videos about proper leg placement? as for remembering where your leg goes, take a day and dedicate it to thinking about your leg. at the walk, trot, and canter, think about keeping your leg in the still position
i always keep my leg at the girth and when possible i always check to make sure it’s still in the right place especially when i’m wearing spurs!