Keep up the positive attitudes whenever around him or talking to him. Make sure you are attractive whenever around him(hair done, nice clothes, perfume). DO NOT bring up the relationship to him, if he brings it up, then talk positively, justify his feelings, even if you don't agree, you don't have to agree, but don't put down his feelings. These are his feelings after all, right or wrong.
Next, make yourself a better person to be around. In the end, you two could end up divorced, but YOU WILL be a better person for all the work you do. Regardless of the end outcome, be a better person for you and your kids, not him.
