my dog is suddenly terrified of walks

hi all! my dog M is a mix, but mostly black lab, and she’s 4. she was diagnosed with hip dysplasia at around a year old. she did physical therapy for a while and they taught us some stretches and things. she also had FHO surgery on one of her hips at around 2 years old, but i’m not sure that’s relevant. the point is, we’ve been told to walk her around our little area of the neighborhood once a day everyday to keep her strong and that it will help stabilize her hip joint. she used to ADORE walks, like would jump up and spin in circles at the door while we got her ready, then would pull the while time with tongue out and a big smile. but within the last 3-ish months or so she has become genuinely terrified. she’ll try to run back to the couch when i go to put her harness on her. then she barely makes it out of the driveway before she goes still, tail between her legs, and then sits down and refuses to go further. if we somehow do make it farther down the sidewalk, she’ll stay tail between legs and try her hardest to pull away/out of harness. i’ve tried observing what’s scaring her and there’s nothing consistently outside. sometimes i think it’s noises or cars, other times i think it’s people or cats. but sometimes none of those things are present and she’s still scared. we’ve tried so many things to help. i’ve tried making her do a few tricks and giving her a treat to distract her and get her to keep walking. i’ve tried waiting it out (like literally sat on sidewalk next to her for 20+ minutes). i’ve tried throwing treats down the sidewalk to keep her moving forwards (works sometimes but is not foolproof and we’re going through SO many treats). i’ve tried walking with a treat in my hand by my side. i’ve tried playing with her inside and then taking her on a walk as fast as i can while she’s still distracted (works momentarily but then she remembers she’s scared). we’ve also gone other places and tried to walk, and while it does work for a while, she eventually gets really scared again. and i also don’t have a car during the day so it’s not sustainable. but literally everything i can think of. the only thing so far that has helped is taking my in-laws’ dog with her on a walk (they live upstairs so usually MIL will join me). but we’re on different schedules and it’s hard to coordinate walking the dogs together every day, and with how strong M is it’s near impossible for me to walk two dogs by myself. and i feel like that also doesn’t address the issue. as far as we’re aware, nothing traumatic happened that would have scared her this bad. but it’s getting bad because i know she needs to walk every day for her hip but it’s impossible to get her to. i feel so guilty and so bad for my poor girl because she’s scared and i don’t know how to fix it. anyone have ANY tips or tricks to help figure out what’s going on/get her confident on walks again?

also: we’re 99% sure she’s not in pain. she goes about her day as normal and still runs in the backyard. there’s no limping. and when we go on a walk with my in-laws’ dog she doesn’t struggle at all. we did take her to the vet at one point, and one of her nails had split so we were like “ohhhh i bet that’s it”… it was not. they got her all taken care of and after recovering for a few days with no walks, she was back to being terrified when we tried to walk her.