How can my ex girlfriend move on soo fast?

we been togetherfor 5 years and we both cheated but i was the one to cheat last. we broke up like a month ago. she met this dude in a phone store and havn with him a week la8r. spendin the night at his house snuggling kissin and everythng..i havnt moved on i still want her back i still think about her and i havnt even had with anybody else. i cant move on. i need answers and advice!