I have a problem gaining weight, too. At one time I was down to 108 from 122 and was getting concerned because I was eating and didn't understand why I was losing. The doc knows that I have panic attacks and am a very anxious person. She said that being anxious can use up calories and make you lose weight. She also said that the more I obsessed that I was losing weight the more I would lose. I was weighing myself several times a day.
Of course, I can't help but worry about everything....that's just the way I am. But, I finally have gotten up to 117 and did it mostly by having a shake in the evening. I can have some dairy if I take a Lactaid pill, so I had Vanilla Frozen Yogurt ice cream mixed with whole Lactaid milk. Slowly I started putting on a little weight. Also, I eat lots of sweets and starches. I never tried the Ensure but it might be a thought.
Believe me, I know how frustrating it is but I guess we have an easier time of it than those trying to lose weight.