Dear Wanderer,
You ask questions that are very common in today's world. When relationships change or falter in this world, it affects our eternities, too.
I have 2 sons that are step-sons, sealed to my husband and his first wife. They have the BIRTHRIGHT of sealing. That means that they will have the protection of being sealed to a worthy family in eternity, whether it is my husband and I, he and his first wife, or perhaps to the families' of their wives. I will be unable to have them sealed to me in this life. If that is what they want in the eternal worlds and I am in a place where I can receive that blessing, then it will be done during the millenium.
While a woman may not be sealed a second time, the same concept protects her. I visulaize it as an umbrella that protects her. If she finds a man that she loves and they marry and the man has been sealed to another woman, they are both protected and those blessings will be untangled in the millenium.
In all of this, it is vital for us to remember that we will have a chance to chose who we want to be with and be sealed to for eternity. Just because a marriage is performed in the temple doesn't set that in concrete until we have finished our journey here and received the approval of Heavenly Father.
I hope this helps and gives you a little hope. Heavenly Father knows the desires and intents of our hearts and will bless us accordingly. He is so kind and wise. He will not leave us in a relationship that will cause us pain or frustration. If we have done all that we can to keep our covenants and things don't work out here in mortality, then we will continue to work on things and grow until the Resurrection. Nothing is set in concrete until we are raised in the Resurrection. So just keep trying to do the best you can and Heavenly Father will lead you by the hand and help you.
Good luck!