Help... i don't know what to do...?

help! dont know what to do with my dog... i've adopted her for almost 3 years and she has this serious skin problem that keeps on coming back. i have done everything i can to take care of her but still the same. it's heartbreaking to see her suffering from pain and irritation. her doctor told me that it's hopeless and if it's ok for me to put her down because its the best thing we can do for her... i love her so much and i don't wanna lose her, don't know what to think anymore. it breaks my heart.