If your metabolism is high it is because you are very active. Or your metabolism is normal and you are not eating as much as you think you are.
People do not have high or low metabolism because they are someway special.
Try to count how many calories you eat each day to establish how much you eat to maintain your bodyweight then increase it slightly until your weight starts to go up.
Milk is good for adding calories or try a weight gain supplement.
Nlarge by prolab is good and not that expensive.
As for your training try full body workouts as you say you made progress with them before. Focus on the compound exercises like bench press,dips,pull ups,squat,deadlift.
The exercises to put the most effort in are the squat and the deadlift. They are the hardest exercises and also the ones which give the best results.