In case of 実感する It means " feel( or really feel ) " ・I felt my happines when I had new family. → 新しい家族が出来た時、幸せを実感した。 On the others hand, 気づく It means " nitice " ・I noticed he arrived at our place when I heard the car stopped. → 車が停まった音を聞いた時、彼が（私たちの場所に）到着した事に気づいた。 If you have any further question, please ask me again.