Needed advice

So I'm in a relationship and it's been on and off for the last 9yrs! We have 3 kids together and I just found out I'm now pregnant again with our forth child! We agreed that we was going to have 1 more just not right now and I was on birth control but then I got sick and had to take antibiotics. I'm thinking that's what caused the pregnancy. But anyways he doesn't know cause I know he will have a fit. Idk how to tell him , hell I'm not even sure how to feel about this pregnancy ! I want the baby but I scared.. Idk what to do !!!!