First, you have extraordinary stress as a member of the miitary that most civilians never experience. It's definetly a factor in relationships. And by the way, thank you for serving. You do us all proud!
That said, I think everyone who ends a relationship has regrets. You think, "I should have done this or said that." And, "if only I had (fill in the blank)." It's also much easier to be objective about yourself, and the marriage, after some time has passed. It is natural to look back and think, "Darn! I was such a jerk!" (BTDT myself!)
So here's my two cents.
First, it's obvious your EX has moved on to a new relationship. I think she might have hoped she would change her mind about you and your relationship when you got home. And, it's an emotional time when a solider returns from battle. Everyone is glad you're home and safe. So, you essentially had a one week "honeymoon" and then the high faded. Reality set back in. She picked the boyfriend over you.
Yes, it sucks. Royally!
So here's your marching orders: Get yourself into individual counseling. You need someone who can help you decompress from active duty AND get perspective on your marriage. You need to do this to be an affective parent to your kids. While you and their mother may no longer be together; you will always be a father, and the two of you will be in contact for the rest of your lives because of your kids. So, if you put your children's well-being ahead of yours and hers, you'll find it much easier to focus on this aspect.
I also encourage you to set goals for yourself, such as obtaining a new skill, taking up a new hobby or reviving an old one, making new male friends, and challenging yourself to grow in different ways. For example, you may want to work on developing your spiritual life, or your professional skills, or stretching and trying something you've never had the time or energy to do to give yourself an outlet for your energy and emotions. (Rock climbing? Sky diving? Fly fishing? Chess?)
I encourage male friendships because it's a very slippery slope to run out and start dating after the end of a marriage. Even though you've been legally divorced for a while, you're not in a great place emotionally to make good decisions about women. I'm not suggesting you must be a monk and stay in the garage! :) But, be extremely careful you don't attach yourself to another lady out of fear of being alone, rejection, or just sexual frustration and wind up with another broken heart (or another child) that you are not ready to engage. (Think like the miltary man you are here. Would you attack a position if you didn't have a clue who, how many, and what the objective was?)
You will survive this heartbreak. When you are truly ready, you will love again. Pain is part of the process. Move through it. Get help with it. Grow through it. A year from now you won't recognize the person you are right now.
Thanks for the advice.
Well, you're about to earn some new battle badges! :) I find it interesting your EX is moving so far away from the "friend." Makes me think he might be her "transitional relationship," unless he shows up in CT with her?