Hello i'm 16 and i have been having problems with gaining weight my whole life . i tried changing my eating program but it didn't help . before i was also physically active and i played various sports . i'm still heavy 123 ibs for 2 years?

If you are trying to gain muscle mass, first focus on an exercise program focused on muscle growth. This will mean regular weight lifting (3-6 days per week) while slowly increasing weight resistance. Focus on eating a clean diet with a calorie surplus of 200-300 calories per day, the surplus calories should come from a quality protein source. Target a protein intake of 1.5-2g/kg of body weight.