Karen,
You don't sound bitter but like someone who has been hurt very deeply. You are right, caregiving is a very tough job and if you have ill feelings toward him that will make it worse. With his history I probably would not believe a word he said. He cheated on you whether he "consumated" the affair or not. Again though I would encourage you to seek counseling of some kind. There are a lot of counselors out there and you need to look until you find one that will at least help you relieve some of the frustration and anger you have. You have a lot of emotion that needs to be vented. At this stage you really need to consider you and your children first and foremost. Your girls are what are important and so do them a favor and get some emotional help before you make a decision.
I am almost 2 years into caregiving and it gets tougher everyday. If necessary can you do that? Probably best if you made that decision before it gets too bad for him. We don't know how long we will be in this mode and it is probably necessary for us to know up front whether or not we can do the job. My situation demands 24/7 with my husband unable to do even very simple tasks for himself. I did not foresee this from the beginning, it has been this way (24/7) for almost a year. Could you do that if you had to? Does he have other family members that could step in and do this?
I wish you all the best and do not envy you the decision you must make.
Ginny