I Weight 20 Stone My Aim Is 13-14 Stone Will My Skin Sag?

The title explains it all but i am 6ft 2 and half, i weigh 20 stone i have many stretch marks round the belly on area but not blood red just pinky colour i have some that are just white, i have long ones under my arm but my arms dont seem flabby, my legs have some marks on the back but not much i am 18 years old i just want to catch the fat before it is to late i did weigh 21 stone 2 pounds i lost 1 stone and 2 pounds some how when i last weighed myself. Just need help a little bit im only 18 just hit me what will happen to me when i get older i got a lovly girlfriend who means the world to me and just dont want her to go off me, self confidence is rock bottom and just need help any advice how i should take this, how to avoid skin sag, bingo wings and what not, i have always been overweight my whole life but just now i have become this heavy like over a period of 2 years. Really need help i dont want to go into depression and stuff because im close but i have family gf and friends that i recognise that care about me so yeah i have a stong mental mind and can do anything if i put my mind at it, well not anything but you know ! Help please :D