Most Helpful Girl
You're upset because even though you've moved on and are happy with your own relationship, you wanted to cling to the other woman as well. No one wants to feel like they can be easily replaced so when an ex moves on, it's sort of a slap in the face. You'll find it discouraging and maybe even experience some jealousy because you feel like she deserves a better guy (you) and she's close with someone else, someone who may not be very good for her. All part of love and war, it will fade with time. Just focus on the relationship you are in, getting frustrated over what the ex is doing will do no good for anyone, especially you and your current SO.