My husband goes soft during sex?

I am hoping someone can make me feel better. So my husband within the last couple months keeps going soft during sex and or can't finish. Like literally inside me thrusting and then goes soft. I ask him if something is wrong or if it is me and he always days no but it is making me very insecure. I know he has no problem when he masturbates and so I just keep thinking he isn't attracted to me anymore. He is only 43. Is this normal? What can we Do?