Aside from supplements and lifting weights, increase your daily calorie intake. Instead of taking 3 very full meals everyday, make it about 5-6 full meals and try to make it balanced and healthy and be aware of the calories. Take note the point here is to increase calorie intake and not merely eating more.
Don't monitor your weight on a daily basis. Try to look at it weekly or better yet monthly so you'll see more visible results. Trying to see your progress on a daily basis and not seeing what you want is major pull in motivation, you'll lose the motivation of gaining weight.