advice for a difficult dog to trim

i have a shih zu and she’s so difficult. she gets matted hair around where she pees and she never lets me trim the hair to make it easier for her. this isn’t a recent thing tho ever since she was a puppy she was hard to trim whether it’s the face or legs or bottom she hates it. but ESPECIALLY her bottom and where she pees. if i go anywhere near her she’s quick to jump or even snap at me. i want to clarify that i’m not rough with her and i do take my time to make sure i’m being gentle enough and from what i’ve seen, i’m the only one she’s like this to. but when she snaps at me, i know it’s her letting me know she’s had enough. but i feel so bad bc i know she can’t be comfortable when her hair mats up down there. does anyone have any advice on how to make her more relaxed when i trim her ?