What wonderful and loving advice you have received already. It must be so hard for you to go through this, and be aware that it is bringing your unresolved issues to the surface.
I have a friend whose mom left her (and her sister) when she was in 4th grade--they came home to an empty house after a weekend at their dad's, how lovely. She is now in her mid-30s and has such abandonment issues, that she refused to have more than one child, because she couldn't stand the thought of having a child who may have felt "less loved" than another, she quit her job that she loved (and had very flexible scheduling with) to stay home, because she couldn't imagine her son not being right by her side, she even planned on homeschooling him because the thought of him being away from her during the day was too much for her-- In other words, she did outwardly things that were probably not in his best interest or her own because of her own fear. Her husband finally convinced her she needed to get some counseling because he really wanted their son to grow up independent and thougtful, not as a tool to help his mom overcome her issues. Her son now goes to Kindergarten outside of the home and she is finally starting to realize that he won't always be there for her, but her husband will... It was hard as her friend to see her go through it, and she really is a great mom, but she was letting something that was hers to deal with become a road block to her very smart and bright and gregarious little boy's social well-being.
The best thing you can do right now is get yourself some counseling. Check through your university services, or county health department if you don't have access through insurance. You will have someone to talk to who is not part of the situation, you will be able to talk about it and be completely honest/open, and you will feel more confident in the long run. You should be commended for NOT letting them see you upset! That takes wisdom and maturity, and they might even sense it to some degree, but you aren't making it their issue. Good for you!
Your kids are so lucky that they have you, if their dad screws up and lets them "know" his GF too soon--it's HIS fault, not yours. If they get upset when it doesn't work out, you will comfort them and be there for them. They will be aware that it wasn't anything YOU did to them, they already know that.
Best Wishes.