My heart goes out to you! You must be devastated. I know, because I'm still coping with my own experience. I wish there were words to take your pain away. It is so hard to cope, especially when it feels like the painful thoughts just won't stop torturing you.
You are right, it wasn't you. These predators have no feelings so he probably has no real feelings for his new woman either. It's part of his game. You know that. What matters is that you are on the road to being free of his manipulation and emotional abuse. You really are better off without him, but it will take time for you to fully embrace that and to let him go completely from your heart and your mind. It is disgusting that he not only treated you badly, but that he hooked up with one of your friends. So typical of a heartless, selfish, psychopath. You don't want a guy like that.
I understand what you are feeling. I don't love or miss the man I was involved with anymore. However, I am still tormented by thoughts of how he victimized me and by knowing he was rewarded for his behavior because he quickly danced into another woman's arms. My guy had at least two women to fall back on. Within weeks of our breakup, he moved in with his ex-wife while I was left alone and emotionally shattered. I wouldn't be any good to anyone right now, so I am putting off dating until I feel more grounded. I won't lie. It's hard to get over this stuff, but it does get better each day. I have more work to do, but I feel stronger and better about myself each step of the way.
Hang in there. It's hard to get over a breakup, but I think it's even harder when you truly love someone who moves on so seamlessly while you are still raw with emotion. It doesn't seem fair when the bad guy appears to get the "happily ever after." You know he is likely just using your friend, but try not to concern yourself with all that. He has hurt you. Period. Dwelling on it only makes you feel worse. Worrying about what he's doing only prolongs the pain. Stay busy, spend time with your REAL friends (sorry about the one that went astray), and take good care of yourself. Talk with people and get counseling if you can. Try to do things you enjoyed before your relationship with this guy. Don't think too much about his appearing to be happy. You know him and you know that his mask will fall eventually. You can only work on what you have control over in the moment, so try to give yourself the love and attention you need to be happy.
If possible, try to remove yourself from any information about what he is doing. It's going to take some work, but you will get through this and be happier in the long run when you find yourself in a truly mutual loving relationship. Feel free to stop back if you need encouragement. This is a good place for people to support each other. You are not alone. Sending you a big hug.

Thank you so much for your kind words. You have no idea how much they helped. I'm feeling better today. I had been doing so much better recently, almost to the point of not even thinking about him anymore, but the engagement set me back. But after the initial shock and heartbreak, and also the doubts that became obsessive thoughts, I'm back to knowing the truth. That he truly is a psychopath, and that he hasn't changed. And that my friend will get treated the exact same way. The sad part is that she has no idea. She didn't learn from what happened to me, or countless other women before her. But, we all have our lessons to learn in life and she's on her own path. It was her choice to not listen to anyone and to think that she was different somehow. Anyway, she's also not really my concern anymore, since she dropped me. My focus now is on continuing my healing process, and your words have helped that. I'm so sorry for the pain you are going through. It's the worst feeling in the world --until you realize that the pain serves a purpose. And that you are a stronger, better person because of the experience. Sending you big hugs back!!!

So glad to hear you are doing better. I completely understand about the obsessive thoughts! They fade a little more each day, though. A new year is beginning, so try to focus on the potential of your new life. That's what is helping me. When I find myself thinking of the old relationship, I try to stop myself and think, "Ok, that's done and I can't change it. In a perfect world, what would my new life look like and how do I get there?" Sometimes, I try to imagine the thoughts of the past as a book I've read and put back on a shelf, or like shadows appearing and fading away. Stuff happened, but now that stuff is nothing but thoughts in my head--those thoughts have no value to me now. In fact, they hold me back, so I try to view them with indifference. Perhaps this approach can help you as well. :-)
I'll keep sending positive thoughts your way.

Happy New Year! Here's to this being a better year for both of us. Your strategy for dealing with the thoughts is great and I think I'll try. For me, I've become very aware of when they come up (which is still all the damn time) and I'll tell myself that I don't have to obsess over it anymore. It's done and I can move on to other things. And then I do move on to other things. I hope the first day of the year was great for you. :)