How can I stop caring about that person?

Start caring about yourself. You are the most important person in your life and you should never come second to anyone.

Often if one forces themselves to stop caring it actually does quite the opposite; It reinforces memories and you subconsciously reminds yourself of why you actually cared for the person in the first place thus preventing you from having a Volta (change in emotion towards someone/thing). I feel that the best way to stop caring or having specific emotions towards a person is not by tackling the emotion ie. forcing yourself to stop caring, but in fact tackling the memories behind it all. You need to make yourself remember these memories without triggering any of the feelings you had when these memories were created. This does sound very difficult and vague but the best way to do this is through keeping busy and surrounding yourself with people whom you actually care for at the moment . Also try remind yourself of how these memories have lost meaning by the person change in actions (im guessing this persons actions were negative?) Best of luck!