We broke up after 1 year of dating, she told me to go and sort myself out, I lost a a tonn of weight, moved apartments, went on a vacation, started yoga, and changed my whole attitude for her, as I wanted her back. I tried the NC rule, it worked, she came back and we chat, and email often, and text. I still love her, she still loves me, she tells me that. but I found out, she does not know, that she is engaged to her ex, weeks after we broke up, she has always been friends with her ex after they broke up and she started dating me. How do I talk about this to her? Do I say anything? Do I just go cold on her and walk away? Help anyone....
Hmm. That's a difficult situation to be in.
The question that lingers in my mind is, "Did she do this with her ex while she was with you?". I wonder this because she appears to be doing this with you, now. She seems to continue the pattern of remaining friends, and hiding her other relationship from you.
I'd ask that question yourself and think hard on it, because someone who does such a thing with more than one man is -not- the type of person you should be seriously pursuing a relationship with. Someone like that will always leave the lingering feeling of wondering if she is doing this to someone else while she is with you, and such suspicion can tear a relationship apart.
It definitely comes across that you love her - you changed yourself so much for her. Talking to her may provide a sort of answer and insight into how she holds her relationships; does she wave off her current relationship because she says she loves you? This leads to the question of why she is still with him, or why she hasn't left him yet, and can lead to deeper problems and answers that will help you come to the right conclusion.
Of course, bringing such a subject up isn't always easy. Even a simple "So when were you going to tell me?" can be misconstrued into a confrontational question that can lead to a heated arguement. Perhaps you could bring up the question of a future between the two of you, and if it's possible. Consider her answers and read the tone of the conversation; then bring up the engagement at the appropriate time. (While it sounds terrible, if you catch her off-guard with the statement, you will likely get a more honest answer from her regarding it.)
By not saying anything to her and just walking away, you leave her not knowing she's done something wrong, and you do not get the closure you need. I would highly recommend that, if you choose to walk away, you tell her -why-, because it's important that she knows you will not stand with someone who can't decide on a partner.
And yes, such a decision will hurt terribly. Even such a cliche line as "Well, if you love her, you'll do what's best for her by leaving her" doesn't ease the pain. My recommendation would be to focus on the life you built up. It sounds as if you've improved nicely, so continue with a new goal in mind: the right one, who will love you and want you (and only you, not you on the side). Throw yourself into work, exercise, health -- keep yourself going, and such a strong attitude will radiate off of you in a good way. The confidence it will build will attract the right one, too.
Now I know that last paragraph sounds as cliche as it can, and there's the simple fact of "I don't want to, because I love her". This is why talking to her is important; approach it, not confrontationally, but as a learning experience. Honestly ask yourself if this is the type of pattern you want to get into, if this is the type of person that you want to have a serious relationship with. And don't be fooled by lines of "But I love you more", or the like. If she truly loved you in the way she says in those lines, then she wouldn't be with the guy she currently is, engaged.
Best of luck to you. I've been in this position before, except in her position, and I sure do not wish this on anyone else after seeing what it did. Thankfully, I fixed mine in time and didn't lose the one of my dreams.