Does accepting the decree of having a co-wife mean I have to stop trying to improve the situation?

Accepting the decree and continuing to strive go hand in hand; contentment with Allah's decree does not mean staying silent instead of improving what you can. You can accept that the polygyny has happened while still claiming fairness, protecting your rights and building your own happiness. Permitted efforts include talking with your husband, seeking advice, looking after your health and strengthening yourself. What needs to be let go of is what is beyond your control, such as other people's feelings or choices. Prayer and tawakkul (reliance on Allah) are the peak of effort, not a substitute for it. Set one thing you can improve this week, and leave the rest to Allah. That is living contentment, not helpless resignation. See existing wife.