Wow, that's a lot of "stuff" to read when there is no punctuation. Next, the correct term for your lady is fiancee. You are her fiance.
Now, down to the problems and there are many. A five year engagement doesn't exist. On The Knot, an online wedding information site: "According to the Knotties who responded to our 2015 Real Wedding Study, the average length of an engagement is 14.5 months—but that's just an average. Some engagements are super short (like a month!) and some can last for years and years. We say there's no "wrong" or "right" length of time to be engaged—everyone's relationship is different, and you and your partner have to decide what's best for you. Here are a few things to consider when it comes to your time frame." Now, all this being said, a five year engagement would indicate, to me anyway, that someone is not ready to be married. What is with this incredibly long engagement? Is there some reason for this or are both of you dragging your feet and really don't want to be married?
Next, on my list of questions is when your fiancee was missing for Friday and/or Saturday, where were her children? Were they with you or did she have them with her? Did this not strike you as being odd? Then to find her at her ex husband's place and to have her lie about this, how can you really hope to work things out?
Now, we get to the part of the story where she doesn't want her family to know that the two of you are trying to work things out. So, she can go out and betray your trust the way she did and then she feels that her family has no need to know that there are serious problems with your relationship. Okay, the family doesn't need to know the nasty circumstances behind the need to work things out, but they do need to know that the engagement is on hold until everything is sorted out.
Now, you are having some serious doubts about the relationship and are arguing about the problems that have erupted. You've been the glue that holds the relationship together and have been there to help her raise her children. For her to sleep with her ex was just wrong no matter how she tries to justify it. You need to put some distance between the two of you and get your life back. You've been used for the past five years and that's a cold, hard fact that's difficult to handle. Get out while you can. To stay in this relationship, if it can be called that, would be foolish. You need to get out, start living your life and hopefully, find a nice woman who will appreciate all your good qualities. This woman does not appreciate you and is using you as a father figure for her kids and as someone who takes care of her and doesn't get anything in return. She can't even be faithful to you! How do you know that she hasn't been meeting up with her ex all along. With the way she lies, you can't be sure of this. Please, move out and let her sink or swim. You don't owe her anything. She is likely getting child support from her ex and should be paying her share of the living expenses. If you are totally supporting her, then you really need to get some legal advice on breaking up with her. Hopefully, you won't be expected to keep supporting her, but since you two have been together for five years, it might be something to which she is entitled to legally. But, check it out to see where you stand.
I really feel for you that you have been so deceived by this woman. Don't let it go on a day longer. Move out, get your own place and if you have joint bank accounts, then separate them. This is where you need legal advice. Good luck and take care of yourself. Don't get into another relationship too quickly since you need time to heal after this fiasco.