Accept that "spot training" is a myth and this isn't a problem you can solve by targeting it. Your belly will continue to shrink as you lose weight, but adipose distribution in women generally means that it will be the last thing to completely flatten. I was a big girl at one time (5'8, 33 to 20) and my "pouch" was the last thing to go. I was angry at it the whole time and it took years.
-it's extra skin so you'd need surgery (I doubt this is it)
-bad posture makes it look worse than it actually is