Bism Illah wa as salaam alaykum wa Rahmat Allah wa Barakatuh.
Time and again sisters raise the issue of the hurt involved in polygyny. They often consider their husbands to be oblivious or insensitive to their pain. However, I ask you to think about the man you are married to. Does he purposely try to hurt you? Does he revel in your pain and misery? Does he smile with glee when you show confused hurt and heartbreak? If you can answer no to those questions, then don't imagine it is any different for polygyny.
For those of you with children, when you had your first child and formed that incredible bond of love with them, could you ever conceive of hurting them? So, what happened when you decided to have a second child? Did that mean you no longer loved the first, or loved them less? Yes, having the second child will take time away from the first, but did you have the second child to hurt the first child and cause them misery? Surely, no. Allah guided you to make a decision to have more children. Allah blessed you with more children.
Just so, Allah guides a husband to have another wife. Just so, Allah writes who that other wife will be. Just so, Allah writes what lessons and benefit he, and you, will derive from that other wife. It isn't done to hurt you; it is done as part of the Master Plan. Allah has sent this to him, just as much as He has sent it to you. Your husband is trying to find a way to follow what he is guided to, and also prevent your pain and reassure you of his love. It's not easy... for anyone.
I could write more, but I want to get this post out. I wrote this in a comment on a different post (thanks, Umm Salwaa), but it deserves it's own space. It's not about what others do to you, it's about how you react. Allah sent it all and it's all to bring us to a better place in relation to Him. This is not exclusive to the wives! The husbands are taking on a lot, and it is not easy for them to cause pain and heartache to the woman they naturally want to protect from hurt and harm. Do you really think they would do it, and put you through it, if it wasn't clear to them from Allah?
Don't question the nature of creation, just take whatever comes and own it. Accept it as especially yours, from ALLAH not your husband, sent as a test and a blessing.
Disclaimer: This post is about the good, practicing, Allah-fearing husbands, not the players who wouldn't know taqwa if it was the only word in the dictionary.