I saw a guy on my track team in college get serious one year after several years of hard partying and a year off from track where he gained maybe 20 pounds.
He came back to the team the next year and during our tough off season conditioning work in the fall, he lost all that fat weight, and his skin was very loose.
My coach told him it would probably take 6 months or more for the skin to completely tighten up, considering it had been stretching out for a few years.
And that’s about what happened. After losing the weight in the fall, his skin was a lot better by the end of the season in the spring, although it still wasn’t completely tightened up yet.
I would guess for you that could be longer. You lost more weight, and had it on for a longer time it sounds like. However, you are only 18, which means your skin is still super young and super resilient.
When you’re 30, you can look back on your teenage years or even your early 20’s and you’ll see how taut your skin is compared to your 30’s.
I would just give it some time. Make sure to stay well hydrated, get lots of your vitamins (like E and C I think are good for the skin), and get a good source of Omega-3’s. All these things are good for your skin and should help its resiliency.
Maybe take photos to keep track of progress over several months. Changes over long periods of time are harder to notice. After one year, you may still feel too flabby, but if you have photos of when you started, you may be able to look back and see “yeah, my skin really has tightened up a lot, I just need to keep being patient”.
Definitely, though, filling out with more muscle will help too.
I would be hesitant to go under the knife, especially at your age. It seems to me, those surgeries are much more for after people are in their 30’s-40’s and beyond, when your skin really has lost a lot more of its elasticity.
I wouldn’t start to consider surgery unless you can see that there’s really been no progress for like a year.