So scared to have the baby

I’m 17 weeks, I have a 5 year old and 3 year old. My 5 year old makes me feel like she’s grown, she’s very independent, very very well behaved, respectful. She can sit and color; read or watch tv for hours. She loves school and activities. She’s a very smart little girl. My 3 year old on the other hand has ADHD and struggles with behaviors. He’s getting much better as he’s getting older but there has been times I’ve just wanted to run away. It’s so hard. He needs constant attention, he gets in trouble all the time and doing things he knows he’s not suppose to. He can’t sit still or do anything for longer then 5 minutes. But now I’m pregnant with another little boy. I really pray having a little brother will help him a lot. I’m just super scared to have the baby. My fiancé works a lot he’s a nurse and it’s gonna be most of the time and I’m started to panic! Anyone else feel this way or have similar stories?