I cannot bring myself to accept this polygyny in my heart. Is that feeling of non-acceptance a sin?

You are not sinning because your heart finds it hard to accept; feelings are not sins, and you have no obligation to pretend to be happy. What is required is not to wrong others and not to break the law, not to force your heart to feel what it cannot yet feel. Discuss your discomfort with your husband kindly, and seek advice from knowledgeable people before making any big decision. If you truly cannot bear it, you also have the right to consider the options permitted by Islamic law, including referring to your rights. At the same time, avoid actions that wrong your husband or co-wife out of anger. Whether you accept it or not, both lie within your rights. See existing wife.