I always over react!

I always worry that my baby boy has passed because I was pregnant with twins & I lost one of the babies! I am so thankful for having Mason still in my belly but I just worry something is wrong all the time I’m 21 weeks & feel him moving sometimes but not a lot so I worry that he should be moving move is it normal to feel worried or at I over reacting my fiancé says not to worry this much but I can’t help it he says it’s not good which I know it isn’t but I need some opinions when did you guys feel the baby start to move for frequently?