If you feel anger, pain, fear or hurt but never show it, you internalize it — you keep it inside.
If you think about the prefix inter- which always indicated something within, you have the meaning of the adjective internalize, "to keep or take something in." It can be used in either a positive or negative way. If you are a debater who internalizes a topic, then you know it inside and out. On the other hand, you're not doing yourself any favors if you internalize your guilt.