getting over him.. need advice?

i broke up with my bf just recently, after 3 months i saw him with his new gf, by the way we work in the same company in the same field, me, him and the new girl, of course all the time i see them, at times i also see him look or glance at me when his girl is not around, but i just pretend i dont care, sometimes we cross each others paths at the corridors or pantry, but i just pretend i didn't see anything i don't care, but why is that i feel sad, i broke up with him coz there's no point being with him, his immature, he never assured me of anything, he just do whatever he wants to and doesn't consider me first before making actions, am i behaving right towards him? by ignoring him even we cross each others path, what should i do to get over him, i know it should not hurt coz im the one who started the break up, but why does it hurt? what should i do im confused,