OP I know how you feel. Please do go to the GP and at least discuss how you are feeling. Even a referral to counselling may be helpful even if you choose not to go on antidepressants. It's not really a case of you failing him or not coping. The first few months are incredibly hard with any baby but with another child to think of as well as the baby being unsettled it's torture.
Don't worry about his weight. My DS1 was born on the 75th centile, went straight up to 91st and is still on 91st now. DS2 was born on 98th centile and went down to 75th. They do tend to change but settle on one which is right for them. There are no rules that say they stick to the one they are born on.
As for the crying, both ours have been terrible criers and really difficult babies. I know it doesn't help you now (I remember exactly how you're feeling) but it really will pass. He won't be a screaming baby forever.
Have you been to the GP with the baby just to check there are no reasons for the crying? Reflux? Milk intolerance? We tried everything with DS1 but nothing worked. We just had to wait for him to out grow it.
I really relate to you crying and wishing you hadn't had another baby. I felt exactly the same when we had DS2. I used to break down all the time and wish that I could go back in time and just not have DS2
. I resented him for taking all my time from DS1. I hated that I was going through it all again. Everyone promised me I'd get an easy baby this time round!! haha. Didn't happen. I'd wake up sweating and my stomach would be in knots at the thought of DH leaving to go to work. I hated every minute of it. DS2 just cried and cried and cried no matter what I did.
I ended up going to counselling which I'm not sure it actually helped but it did give me someone to be honest with and explain how hard I found it. It was nice to have someone nod and not judge me when I told her how I felt. I am prepared to be flamed for admitting this now but I remember telling my counsellor that I wished DS2 would die of cot death so I didn't have to live this nightmare anymore
. I am not trivialising how terrible it would have been if it had actually happened and I am not proud of what I said but that is how low I had got. How desperate I was. I knew I'd never hurt him but I believed that he had 'ruined' our family. My counsellor was extremely understanding and helped me work out strategies to make me feel like our family wasn't ruined, it was just different to how it was.
I feel so guilty for the times I'd shout at DS1 as a baby and cry and wish I'd never had DS2 but I know it was just an horrible situation. It wasn't that I failed them as such. I was doing my best but I was depressed and they were extremely difficult. Like I say, I never wanted to hurt them, I just didn't want to deal with them and the incessant crying and particularly with DS2 I felt so much guilt for DS1 and everything he was missing out on because of the crying baby.
It really does get better and you will be OK.
One day you will look at both of your children and you will be so glad you had both of them. You just have to survive this part. Sorry this is so long, just wanted to let you know you aren't the only person to feel like this.