Help for Unwanted Underweight Problems

Being underweight is NOT stylish, and the quote, "You can never be too thin" is rude and promotes young people to diet until they're sick! I am 5'2" and weigh only 89 lbs. Clothes don't fit, my skin is "drooping" and I feel tired, run-down with no energy to keep up with others. I am not suffering with anorexia and I want to GAIN weight in a healthy manner. I don't want to down Big Macs or fast-food to pack in calories. And, I hate it when people say, "I wish I had your problem!". Or, they gossip and say that I'm anorexic, or they think that I just never eat. I eat all of the time and drink lots of Ensure, but I still can't seem to keep any calories in my body. Tests have ruled out any type of disease or thyroid problems. Can anyone comment on this problem or let me know of any healthy ways to gain weight. It's NOT fun and I'm starting to look like I'm 90 years old, (I'm 54), with bags under my eyes, sagging skin and my face looks skeletal! Any help would be greatly appreciated!!