Your English is perfect. Unfortunately Dove, I have bad news for you. .
Sagging breasts are due to the loose supportive "structures" inside them.
If the skin is loose in
texture, that can be partially fixed by a dermaroller but there is no way a dermaroller can provide a breast lift.
A dermaroller will improve your stetchmarks and it can tighten the skin texture to a certain point. A dermaroller has an effect on the skin only. Meaning: To a depth of 2 mm.
If you are losing weight, the fat is disappearing from your breasts as well (unfortunately) and it makes them look loose.
So sorry - I do not know any method apart from surgery to achieve a breast lift..
Breasts themselves have no real muscles - they cannot be lifted by exercises.
Breasts consist mainly of fat.
There is a new, interesting method of enlarging breasts with specially treated autologous fat. Fat has been used as filler for many years but it never lasted long in the body. It was absorbed. The new method mixes the fat with stem cells and the result is very long lasting. It looks totally natural.
Here are some photos form a Japanese clinic:
https://http://www.cellport.jp/english/example/list_bust.html It is rather expensive at the moment but it will eventually get cheaper. You may consider it in the future.