You're not just a bad sleeper, and it's not just your weight or your age.
When you fall asleep, your jaw relaxes and your mouth opens, causing your tongue to fall back. This means your airway, which should be delivering oxygen to your brain and body, narrows or collapses entirely.
Your body panics and jolts you awake just enough to gasp for air, which is why you wake up feeling like you've been hit by a truck.