You're 18. You can look forward to several more years of perkiness. :) A loss of ten pounds will likely reduce the size of your breasts, but I really doubt you'll sag.
One way of making sure is by doing exercises that strengthen the pectoral muscles. Firming the areas under and around your breasts will help to keep them happily in place. Push-ups (even wall push-ups) and doing "butterflies" with five to ten pound weights are two ways of strengthening the pecs while keeping a girlish figure.
I don't think they'll necessarily sag, especially since you're so young. I think they'll just get smaller.
It won't sag because you're only going to lose 10 pounds. The only way it will sag is if you are totally overweight and your skin on your breasts have already stretched. This happens a lot on obese people who loses hundreds of pounds.
If you take a look at their bodies, the skin sags. Even obese men who lost a lot of weight will have saggy boobs.
The sag you would experience would be from gravity, in essence gravity would use the natural inherit weight contained in your breasts to pull on them over a period of time, with the effect being sagging, so you can lose the weight still, just the effect of your concerns would be from a totally separate cause altogether. Forget that advice that they will get smaller, the fact of the matter is you have bigger than normal glands, for one reason your glands within are of a more higher count, meaning you have more mammory glands than a person who has smaller ones, something along those lines, can't remember exactly, but do some research I am sure you will find what I mean, and it will be pretty close to the bulls eye.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.