First of all, you're not fat. It takes a while to feel comfortable on a skateboard. I'm a guy, 6' tall, about 150 pounds.
Okay, if you really want to learn how to skateboard, you need to get one. Riding takes about 5 hours to comfortably do, so you need to take the time, and it will get easier and easier. After 2 weeks I still was scared of steep hills.
You need to do things step by step, like every skater does. First ride in a straight line, then learn to kickturn. Then you're ready for an ollie. This, depending on how fast you learn, could take 5 days to 2 or 3 weeks. Also, it's easier to skate flatground, so try that. And usually, younger people, like for instance your cousins (I think), learn to skateboard easier, and are "fearless".
So, it's not you, and you're not fat! The message is to go skate, everything takes time to learn.