You might find this website very helpful. It's called My Body Gallery. a woman created the website to show how different women's bodies can be.
I'm not sure of your height. For not, I inputed 5ft 6" and 150 pounds with an apple shape. If you check out this link, you'll see pictures women have posted. They are ALL different even though they are the same weight and height. www.mybodygallery.com/search.html?height=5
Will you ever have a perfectly flat stomach ? After multiple pregnancies, it's not going to be an easy thing. You may have to consider a tummy tuck if strength training and a healthy diet don't get you where you want to be. Cosmetic surgery is always an option if you have the money.
I know I will never have a perfectly flat stomach. When I lost weight, I ended up with sagging skin on my stomach. Can I wear a bikini ? I can, but it wouldn't be pretty. LOL !!! The point ? What I can with my body since losing the weight is more important to me than how it looks. I'll never have a flat stomach and I'm okay with that.
Do check out that link and look at the photos. It's really interesting.