What happens to my metabolism if I consistently eat too little?

Contrary to popular belief, nothing truly significant happens to your metabolism at anytime except in severe starvation when your body fat would be EXTREMELY low. Most people (even trained professionals) believe they eat fewer calories than they actually do, which is the reason most people hit a plateau. If you need to lose weight and you are not, eat less and/or move more and forget about slowing your metabolism or the amount of calories you think you were eating. That said, the point is not to lose weight too fast by drastically reducing calories. This is because, first of all, that method is miserable and generally not sustainable.

Second, although there is a slight (but again, insignificant in the big picture) metabolism down regulation in response to a very low-calorie diet, the main reason it may appear to slow down more than it actually does is because the very low-calorie intake is slowing YOU down. In other words, you become tired with less energy, causing a reduction in your daily activities, causing you to move less thus burn fewer overall daily calories.