In a months time i will have to work with someone who i deeply cared for but they hurt me. How do i behave when we come across eachother?

Try to be as professional as possible. Remember, when in the work place, you are there to do a job and as hard as it may be, try to focus on the goals of the task and not them. You've got a month till you see them, so in the mean time try to get out and focus on the things you enjoy the most. Once it's time to meet to work try to treat them as a work colleague, nothing more, nothing less. Although it may hurt to see them, try not to worry about the past, you're a new you and you've got your own aims and ambitions in life.