Not sure what you mean "how to be heartless" but ill just attack it with some knowledge.
Just realise that emotion and logic do not mix, listen to your logic first
How many times have you done something cause you "FELT you had to do it" but you knew it wasnt going to work or was bad. And it did end up bad.
Anyway on a different note
You shouldn't care what people say, because who cares it's their opinion. you are BETTER than that to let yourself waste YOUR energy on THEIR opinions which do NOTHING to you unless you let them.
basically be confident, strong and truly believe in yourself, and you will not be bothered by others, and make the life you truly want for youself.
On another note
Perhaps you meant "how to be strong emotionally"?