I don’t want kids

How do I tell my boyfriend that I don’t want kids when I was 23 I lost a baby which caused my first divorce my boyfriend knows this and he’s been asking if I ever wanted to get married again or if I wanted to have kids and I told him I would marry him in a heartbeat but said nothing else I don’t want him to have what happened to my ex husband if we were to ever lose a baby my doctors told me that it wasn’t my body it was just that the baby had a heart problem that wasn’t detectable but it just makes me worry about having anymore kids I’m 35 and I can’t push it off anymore some give advice I need it