Hey, I can give you a little bit of advice as I am in a similar boat. I have difficulties gaining weight but I have noticed a couple of things that have helped me. I have a fast metabolism as well and just encourage you to stay consistent and don't give up! 1. Increased your daily intake of calories, make sure that you are taking in more energy than you are putting out. Try things like protein shakes, smoothies (loaded with fruit and raw nuts/seeds), porridge, grain etc. 2. Weight training - If you are not going to a gym I would suggest trying that out as well. i have not been going for very long but have already noticed a difference in my body. Gained weight slightly. (Once again be careful that you are consuming enough food throughout the day). Anyway, just a few pointers but I am sure the others on here will have much more to say than me. if you have any questions feel free to message me!