How can I stay fuller longer?

it doesn't matter what I eat today I cannot feel full, I eat a lot, and my stomach still growls all day. today I've had 2 bowls of cereal, a cup each of watermelon and honeydew, a big cup of coffee, snacks (cheese string, pringles sticks, almonds) a turkey sandwich and a snickers bar. why won't I stay full? usually when I eat that much I am stuffed ( I usually don't have a huge appetite) theres still diner tonight but still what can I do to stay fuller longer?