Polygyny is halal, yet I still feel betrayed. Is this feeling normal?

Your hurt feelings are valid, and they do not contradict acknowledging that polygyny is permitted. The ruling explains what is halal, but the heart still feels a great loss and change. Acknowledging these feelings is not a sign of weak faith; denying them only makes them heavier. Put a name to what you feel: sadness, fear, or feeling unappreciated. Pour it out to Allah and, if needed, to an understanding counsellor. At the same time, avoid condemning your husband as a traitor if he is exercising his right honestly and fairly. Two things can be true at once: this is halal, and this is painful. See counselling.