What does "I can't help it" mean?

When someone says “I can’t help it,” or "I can't help doing it,” it’s a way of saying that they don't want to do a particular thing because they know it's not good, but they can't stop themselves from doing it. 

People use this expression when they are having trouble breaking a habit, like biting their fingernails or eating too much sugar. They also use it when something is external and beyond their control.

• I know I shouldn’t eat sweets when I’m stressed, but I can’t help it. 
• I couldn't help staying up to watch the end of the movie.
• I can't help loving you, even if we can't see each other anymore. 
• The children can’t help it. It’s not their fault that they were born into a crazy family. 

I can’t help it + if is can be a way of declaring that something is not your fault, as in:

• We can’t help it if there is a lot of traffic. There’s nothing we can do.

I hope this helps.