Please help my dachshund?

i had her for 4 years. ever since i had her shes had an eating problem. when i saw her at the pet shop she was underweight but i bought her, i felt kind of sorry for her. i had her dewormed and all that stuff. she ate smal amounts of dog food but ate more of her own poop. STILL to this day same old thing. i have switched her dog food brand over last few years, tried wet canned food, but that gives her severe diahreaa, the vet said nothing is worng with her health to cause her not to eat. she will eat ppl food but is very very picky at what she takes. she will enjoy cat food for a day or two, but gets sick of the taste and gives up on that too. i even tried just only leaving dog food out for her for a few weeks. but she would only take a few pebbles once a day, she dropped weight. im tired of this, what can i do? she hates dog food i tried everything. she wont eat it moistened with water, or dried, and canned food gives her the runs everytime. i dewormed her again, still same old problems. vet didnt do nothing either. what can i do???