I am not saying you need to exercise more, I am saying you obviously need to exercise differently if you don't get the right results.
Spinning is great, but will not give you a flat stomach, you are not exercising the right muscles. Weight training is more than recommended for women, but again, I don't know what muscles you are training there! You can get a fantastic butt and not exercise your belly enough, if that is your goal.
Core exercise are great, but how long for? I am not behind you, but clearly you don't do enough of what you need.
That is exactly my point: if you do not exercise specifically for what you are aiming for, then it's disheartening if you don't get the results. Of course you need an all round exercise, to tone everything, but if your issue is your belly, then you need to insist more. Yes you can have looser skin, but the overhang is not a curse. Yes you can fix it with exercise (and diet).
At the end of the day, it's up to you. Believe your belly is gone forever, or accept that the right exercises can help you and find a personal trainer (or a class or whatever) that can put you in the right track.
I have seen all the excuses: no time, no energy, woman who exercise neglect their kids, not enough money, no one exercise one hour a day. It's entirely up to you. I am not against plastic surgery, but I prefer the natural way. It takes longer, but the results are long lasting and safer.
Back to your original post, it's worrying that you make comparison with a 5 year old.