How can I stop being cold, callous, arrogant, indifferent?

I can think of all sorts of nice things I could've done or said to ppl I like, but only when I look retrospectively to my actions. How could I be able to think it in real time,so that I be able to act when needed?

Think about the situations where you could have been more empathetic, and apply what you've learned to future situations. It's okay to pause a moment and decide how to respond to something. You don't have to say the first thing that pops into your head. Put yourself in the place of the person you're with and think about how you would want someone to respond to you.

Practise stopping being often. Respect others for what they are and what they stand for.
look inside you
Perhaps if you find out why you are so cold, callous, arrogant and indifferent, you can change. I went through a phase like that and basically it was because I was afraid of being hurt. Knowing that I could talk to people without thinking that anything I say to them would put them in a position to hurt me. I am the only one who can really hurt me (we are talking about mental & emotional abuse, not physical).
If this outlines your basic personality, you may be what is called a psychopath, now called anti-social behavior disorder. Google the term. Find the site that lists the hallmarks of this condition. Believe there are 5, 3 is a diagnosis. From my friend who is a counselor, it is about un-treatable, and never curable... basically, it is a personality defect. It is as if you were telling someone 6'5" , "don't be tall". not gonna happen.