How to gain weight that will stick

ℓєχιι 🦄 • Boy mom👩‍👦• God First 🙏🏽
So I've been 124 pounds ever since I was 16 , I'm 20 now. I've done everything to gain weight. I eat like a pig, I drink boost protein shakes, no matter how many pounds I put on their gone by the next day! I was told I have a fast metabolism but I really would love to gain at least 5 pounds and have them stick! Any tips/advice?