Not sure what I should be feeling

I'm 20 weeks pregnant with my second child and I've honestly never felt so low in my life. My first pregnancy went very smoothly, no sickness and aside from a little sciatica towards the end no problems at all.
The problems started when my 1st was 8 weeks old, he would scream from 7 am - 7 pm every day (this lasted 6 months) and only ended when I personally did enough research to find out that he was intollerant to cows milk, he also had colic and reflux. Doctors finally confirmed it all after I'd done all the leg work! Once we put him on other milk and medication for the reflux things improved drastically. He's now a (mostly) happy 2 year old, although he is very demanding and will not sit still for a second.
My problem is that I'm terrified that the baby I'm carrying will be the same as my first and though we thought about this before planning to get pregnant it didn't sink in that it might happen again until I saw the scan picture. I ended up in floods of tears and unable to lift myself out of a bad mood for weeks after.
I keep thinking I shouldn't have done this, I should never have risked doing it all again and now I've left myself no choice. The lack of energy that naturally comes with pregnancy has left me with less time and patience with my 2 year old and I'm feeling like a bad mum already so adding more to the equation terrifies me.
I constantly feel out of sorts, down in the dumps and worried about what I'm going to do. My friends with kids keep banging on about how good thier kids and how they sleep through the night and don't play up which only serves to make me feel more useless (mine didn't sleep properly till he was 6 months old and that was only cus we did controlled crying).
I just want to know that I'm not alone, none of my friends seem to have felt like this and although I have a very supportive partner (who has been very patient and reassuring) there's no one I feel is in the same boat that I can talk to.. Just wanted to speak to someone who truly understands.
Thank you