Dic: can't help it, couldn't help it

(I) can't help it.

1. There is nothing I can do to help the situation.;
2. That is the way it is.;
3. There is nothing I can do.
   
   (Often in answer to a criticism.)

• Mary: Your hair is a mess.
  Sue: It's windy. I can't help it.
• Fred: I wish you'd quit coughing all the time.
  Sally: I can't help it. I wish I could too.

(I) couldn't help it.

1. There was no way I could prevent it.;
2. I was unable to prevent something from happening.;
3. I was unable to control myself.

• Sally: You let the paint dry with brush marks in it.
  Mary: I couldn't help it. The telephone rang.
• Fred: You got fingerprints all over the window.
  Mary: Sorry. Couldn't help it.

.... McGraw-Hill Dictionary of American Idioms and Phrasal Verbs,
http://idioms.thefreedictionary.com/help