YOU CAN'T HANDLE THE TRUTH!!!
Oh, uh, sorry. Just an involuntary reflex. I watch a lot of movies.
Okay, you want the truth? Here's the truth. It's not that we don't notice your flaws. Of course we do; we're not stupid, either. It's that we just don't care. We love you for 1,000 other reasons that are infinitely more important than a bit of flab: your heart, your sense of humor, your intelligence, all that kinda stuff. The good stuff. The stuff that matters.
Sure, guys like women with fit bodies, but how can that be a deal-breaker when few of us have perfectly fit bodies ourselves? Keep in mind that I am speaking from the point of view of a fully-grown adult male, not a 19-year-old, and I'm talking about the difference between mature love and immature love: surface things like appearance lose importance as you mature and fall more deeply in love with someone.
Let's say you had a boyfriend you loved because he was kind and fun and treated you like a queen. Then one day his hair turns gray or he loses a limb or gets a big ugly scar on his face from an accident. Is that an instant turn-off? Do you immediately fall out of love with him because he's now flawed and disfigured? Of course not. You don't love him for his looks, even if he's handsome; you love him for what's inside.
I've said this before: women are much harder on themselves than their lovers are. You might cringe, but he wouldn't rub his hands all over your body if it replused him. The guy obviously loves you for other reasons. And again, a little flab - SFW? Whose body is perfect?
Physical appearance matters more at the beginning of a relationship than later. We are attracted by looks, then we get to know the person and either fall in love or move on. I don't know your age or how long you've been dating, but if your guy didn't like the way you look, seems to me he'd be long gone. It's obviously not an issue for him. So why is it for you?
If the flab bothers you that much, do some crunches. Otherwise, be happy with who you are and don't worry about what he's thinking. If he would dump you over a little flab, would you really want him anyway?