I had twins 24 years ago - I was quite thin and only got 1 stretch mark with DS, and my belly button never looked quite the same - but with the twins, I had stretch marks all over and stretch marks inside my belly button 
which vanished, amonst masses of saggy skin. I despaired. There was a 'helpful' doctor on Womans Hour who said if your tummy had not gone back after a year, then it wouldn't. 
However! Every year saw an improvement. I took up ballet after 10 years, and my belly button came back 
A lot of it is making yourself tall and holding everything in. I would recommend doing physio, or pilates, or ballet, or contemporary dance, or yoga, as they all improve your core and your posture. At least once a week. And anything that increases or maintains your general fitness on top of it. This all makes you feel better, healthier and gives you a sort of body confidence that is bigger than a saggy tummy.
Now, it looks ok-ish as long as I do not wear anything that cuts across at bikini height, when I get an overhang (low bikini knickers - I wear midis, and low cut jeans). I usually wear a cotton vest tucked in, under things too. (I can look rather good in clothes 
)
Like Madlizzy, I have grown into it and regret nothing.
( 
for Madlizzy - I am in awe - 3 babies and only two knees!)