how to act careless and cold ?

Don't be cold or unfriendly to someone who's not showing much interest in you. Instead, be very friendly, tell him you'd love to see him but you are just way too busy. Then make it true; get busy! Find other things and people to distract you from brooding about him.

That isn't something you can be taught. It's something that you are either are or you aren't. I'm cold and heartless. I don't care about who I step on.

you hate your feelings and not the person who made you feel that way? oh dear the right guy will never leave you feeling that way.

Look at Hillary Clinton

It's very easy. I do it all the time.