I want to encourage you to keep up the exercise, but please realize you can't spot lose. In other words, if there's fat on your body, you will have to actually eat healthy foods, consume less calories than you burn and keep exercising. Those situps will strengthen your abs, but it won't melt the fat away.
Stretch marks are scars. In time, they'll fade, but will never totally disappear. The peddlers of cocoa butter would love for you to believe that you can make them go away by slathering it on but in all actuality, what you end up with is a high moisturizer bill and a very soft belly.
All of those weight loss commericals are are just that, commercials. They're selling a product and are surely not going to tell you it's a struggle to lose weight and that those smiling "success stories" had to really work at getting their pre-baby body back. Hang in there! If you set your mind to it, you CAN make it work for you. It takes hard work, though.