It is normal for there to be complex emotions in these situations. Even in narrations, it says that a woman's jealousy over her husband is due to her love for him. This is apart from other concerns, such as about the financial effect.
There is nothing wrong with acknowledging the reality of your emotions. In any situation, is healthier to work through things such as anger or resentment because negative emotions can eat at us, and we are the ones who will suffer. However, this takes time and there isn't an instant fix.
Bringing another person into the picture (either in terms of a second wife, or illicitly) also changes the relationship between two spouses, and it takes time for the relationship to adjust and to find a new normal. Also, sometimes it changes our own sense of identity and the way we relate to the world, since marriage is heavily tied to identity, and so sometimes it takes time to develop a different sense of self if there is a change in marital life.
There is a certain wisdom in accepting that we cannot control other people's decisions, and to accept that people in our lives will do things that we wish they didn't. Allah only takes us to account for our own actions and choices.
Ethically, it is good to treat correctly the other woman in the same way one would treat correctly any other person, especially if she is a sister in faith.
In societies which are not structured to support polygyny, such as many urban areas today, and where it is a burden for a man to be equally responsible to two separate households, second marriages often don't last anyway, especially if the second wife is getting the lesser end of the deal (for instance, supporting herself and her children financially while the first wife is a housewife). (This is especially the case when the first marriage has been established for a long time and is not ending, and there isn't a strong reason pushing the man to take an additional wife.)
When the Prophet (S) and Imam Ali (A) had multiple wives, they didn't have to commute 4 hours in bad traffic after a 10 hour work shift and pay insurance and mortgages or exorbitant rent for both families, etc. The culture also was set up in a way that acknowledged polygamy, whereas some modern cultures are not even favorable to having one wife and child-raising, let alone multiple wives. Of course, this varies tremendously from place to place.
Sometimes second/third/fourth marriages do last, and I don't want to give you a false hope, but just putting this out there. Time shows the end result of all things.