How can i get my dog to eat?

I have a bichon-frise dog. At a healthy weight she should be about 15 lbs, but she refuses to eat. She usually hovers at about 7 - 8 lbs. she looks absolutely emaciated and it is heartbreaking because ive tried everything to get her to eat. Her ribs poke through are all visible and she has absolutely no fat. i leave her food out so che can pick at it as she please. ive tried many different kinds of food but nothing seems to entice her. the vet suggested putting olive oil on her food and that didn't help. she usaully eats about 3/4 of a cup of food a week. a few weeks ago i had to have her fed through a tube at the vet hospital after she collapsed. this problem has been getting worse and worse for the last 4 years. She isn't an old dog and she is not sick. i don't know what else to do. please help!