If you have had her a year and she still doesn't trust you enough to accept a leash and collar, it may take more than the above suggestions. A harness still has a strap that goes around the neck, and as different as it seems to us, it may still be a collar to her. Do you have an area where you can can safely let her walk off lead? You may even need to carry her back away from the road before putting her down. I have found dogs that would be gone in a flash off lead at home will stick to you in strange places. Take the leash and collar with you. Let her associate the fun of a walk with you with them. Perhaps at home, leave them lay near her food and water dish. Move them to by her bed or crate at night.
Do you have a friend with a dog that would allow you to walk it while she watches? Perhaps let her see a group of people with their dogs enjoying being on lead.
I seldom suggest a professional dog trainer or behaviorist, but this may be the severe case where a good one may be the only answer. Unfortunately finding a good one may be difficult. You could start with asking your vet. Perhaps a Lhasa rescue group would be interested and able to help. If she came from a good one, they would have solved t problem before placing her with you. You may find a rescue near you starting at
American Kennel Club - Breed Rescue dog dogs puppy puppies
Try improving her bond with you through these exercises:
''Elevation for small puppies: Sit on the floor and gently put your hands around your pup's middle, below his front legs, and lift him up. He is facing you. Hold him for 15 seconds. Repeat until he no longer struggles. If he is past 10-12 weeks, lift his front feet off the ground, but don't pick him up.
Cradling for small puppies: Hold your puppy gently on his back, as you would cradle a small baby. If he struggles, hold him firmly until he quiets for 10-15 seconds. With larger pups, you can do this as your sit on the floor, with your pup between your legs.
Quiet lying down: Place your pup on the floor on his side, with all 4 legs pointing away from you. Use your hands on his neck/shoulder area and middle, to hold him in this position. When he is quiet, praise him. Lengthen the time that you keep him quietly in this position. When he accepts this position well, handle his paws and muzzle, while keeping him quiet.''
The quotes mean this isn't my original work. It is copied from my Puppy Raising Manual. I have long used these or minor variations of them, and they are very effective. You may want to give him a belly rub while he is on his back too. Helps bonding. There is a big difference between him rolling over and demanding a belly rub, and you choosing a time to roll him over and rub his belly. The latter cements your place as pack leader.