Gaining

I want my belly hang, but when i ever reach that state will my belly ever be flat again if i try to lose weight then. Or will my skin be lose for the rest of my life?

When you gain weight, your body makes more skin to cover your bulk. When you lose weight, you do not lose the skin. So there will be left over skin if you drop the weight.

There is skin removal surgery, so you have that option.

I'm no expert or speak from experience, but I know of a couple of big guy friends that have lost a lot of weight, and from what they've told me, it depends a lot on the speed of weightloss and how much you've lost.
Think anything over 100lbs of weight loss, you will have a hard time of having your skin going back to it's former state.
Also when losing weight it shouldn't all be cardio, which would ensure your skin to sag more, but doing strength training as you lose weight in the meantime. Should always be a balance of the two.