Maybe?:
You are a radiation particle of sorts?
My life is so short... too short, indeed.
Radioactive particles can have very short half lives
However, I like using it to help people live better.
Chemotherapy uses radiation to help cancer patients, X Ray's help see things we can't on the surface.
At least, this is what I was made for!
Not sure what is defined as made, because if the roots of radiation are traced back, then we see a rather destructive history, but I guess if we go with the medical side of things, it is produced for good.
Since my countless brothers and I were born, we've been called fat :(
Not sure, maybe the particles are rather spherical and large?
But the satisfaction of opening new doors wherever I go makes up for it!
Cured person is a new person, a new person which opens doors to a new life
Hint 1:
Although you can't see us, you would normally find us in a rather spherical geometry. (That's why people say we are fat)
Radioactive particles are spherical
Hint 2:
We hate thick obstacles, they make us feel tired!
Thick materials can slow down or completely block radioactive particles
Hint 3:
We are a product of engineering advancements, we owe our existence to them.
Engineering advancements allowed for the creation and emission of radioactive particles
Hint 4:
I fly!
I guess they fly?