It seems like you've left, but in case you're still checking in here I would have two pieces of advice.
1. Be as honest as you can with everyone involved, including yourself. This doesn't mean giving your GF unnecessary details of what happened, it does mean letting her know that you work with the other woman.
2. If it is yours, be a father. When I made that commitment it made me a better and happier person. I believe it has the same effect on most people, although your situation certainly is beginning from a more difficult place than most.