Hello, I believe I can help you with this...but first I want to say how much I feel for you in this by way of the feelings you are left with to try and deal with whats happened after this10 year relationship.
It sounds to me as though the woman involved is carrying a lot of emotional baggage from one relationship to another at the expense of not only her former lover but in particular you...The sad
thing about her dissolving the relationship you had, is the very fact that she did
not talk to you about her feelings and what was going on for her..this is no easy thing to accept by all means, particularly if she is now considering the affect it will have upon her present lover/relationship...and not the affect its having on you.
I would like to invite you to reflect a bit more on your own feelings for a moment :the grief hurt /disappointment/ anger/ bewilderment and many more..that will be associated with your depression through the loss of your relationship.
If you evaluate further your feelings and the thoughts that you might have with great honesty, what are they based on (1) the issue of her not telling you (2) her going back to her former lover or (3) or thinking that you weren't good enough for her/ ?
This might tell you more whats really going on for you inside ..so that you can pinpoint it and deal with the outcome better emotionally.
You also need a chance to then 'express' to her exactly how you feel about things..) if you can't do this with her..then wring them out in the form of a letter can be very freeing and therapeutic ..for you...but you will need to get them out in the open..
you might even wish to actually send the letter to her so that she knows of the hurt that she has caused you.If you decide not to send it but burn it then that's ok to ..the release will still be very freeing for you.
There is no easy solution to what you are feeling at the moment accept to experience what you are going through..& accept your feelings ( because they are all very real) work through them and' with' them ) this takes time.. if you shut them down they will continue to resurface and prevent future healing until you accept them and whats happened ..
You might well need to see a very empathetic/understanding therapist/counsellor in your local area just to talk about it and perhaps seek medication with regards XXXXX XXXXX depression just to help you up and over this very difficult time.
I hope that my response has been helpful to you..please accept it if you have no further queries.
My thoughts are with you in this.