142nd reason not to date you co-worker...

i got some bad news at work today...a good friend of mine quit...the problem with this is that we shared a lot of responsibilities and helped each other out...our work overlapped in some circumstances...i knew that the news of her leaving meant a double work load for me...there was no doubt that they wouldn't replace her...just expect me to work double...this isn't the bad news...not even close...i got put on a project she was working on...with my ex. i have had to do actual work for him for about an hour in the past 3 years...until now..300+ hours over the next 4.5 months. We are both adults and I do realize that, but I do know that I hurt him and wounded his spirit and I also know that he isn't really over it...we haven't spoke, as in had an actual conversation since the beginning of february (with the exception of 2 cold sentences he IMed me today)...i have said before i do still feel a little guilty for the way things ended, but why this? why is it the past can never stay in the past and that it's always coming back to haunt us? why is it always the one most random thing that you never want to happen always happen? is god out to get me? is he forcing me to deal with things in the past that are better left in the past? do i let it go, keep the next 300 hours of our working experience completely professional without any chit chat about what we are doing on the weekend? what's with the constant drama?