16 weeks 3 days and freaking out

Hey ladies im 16 weeks with my second baby. My first child is 2 almost 2 1/2 and she doesnt understand that we are having another baby. She jumps on me and im afraid of losing this new baby. Ive told her over and over that mommy is has a baby growing inside her but i dont know what else to do. She hits my belly all the time. My next appointment is next week to hear the heart beat im afraid one day i might not hear the heartbeat.