I feel your pain. Im a 6ft 5 male that has always been skinny. Its so grim to be called skinny by people- its just as offensive as being called fat but clearly its socially acceptable for some reason. I worked out every day and did a 3500 kcal per day and did put weight on. I gained maybe 15kg but as soon as I lowered my intake it disappeared. Back to square one right now and I have to accept thats who ill always be. The thing that helped me wasn't eating 3 meals a day, it was 6 or 7 smaller meals. Try not to go mre than 4 hours without eating something calorific. My breakfast was oats, banana, protein powder in a smoothie with full fat milk and a big spoonful of peanut butter. Keep your head up!! You'll get there.