How to gain weight?

I have been trying to gain weight for years, since I was 16! I’m now 24 and still under 100 pounds, ugh! I’ve tried everything! From taking vitamins, eating whatever I’ve wanted, exercising and still nothing! When I do get to 100 pounds, I get so happy and then I loose it. It gets so overwhelming because no matter what I’d do, I always go back between 95 to 97 pounds. I used to be 118 pounds, at my heaviest and healthiest weight I’ve ever been at my whole life! If you’re wondering why I’ve had a huge weight lost is because I was going through gallbladder attacks and my doctors didn’t know it til I was 18 or 19! Had surgery and no more pain! It’s been a huge battle for me! I just don’t understand how or why I’m not gaining any weight when I’m needing bigger clothes now. I used to be in size 0 and now I’m in size 5 or 6 in pants and from x-small to a medium in shirts and still 95 pounds. Lots of my family says I look better than before. Yes I agree! But I don’t liked that I’m still under 100 pounds! 😔😥