I struggled going from 1 to 2. (I actually struggled with the baby phase first time round actually, but second time round with having a 4 year old and a screamy newborn, was worse. Almost unbearable!)
Please remember you aren't the only one who has/ is feeling this. I remember everyone telling me it would be much easier with our second, I'd know what I was doing, baby would be more relaxed, I'd find it all would come so naturally.
In reality it wasn't like that at all. It didn't come naturally. I found it all very stressful. I'd forgotten a lot of the baby stuff with DS1 and found it very very hard going back to sleepless nights, nappies and hours of soothing a tiny baby indiidual who I didn't know yet.
It was all made worse by feelings of guilt about missing my time with DS1 and worrying about ruining his childhood. Not to mention that DS2 cried all the time so this cute little baby brother DS1 had been expecting, turned out to be a howling, non sleeping, stress inducing ball of colicky rage.
I remember crying a lot and trying to hold everything together. Trying to make DS1 feel like nothing had changed when in reality everything had changed. It was horrible and I went into over drive trying to make DS1's life 'perfect' so he didn't feel he was missing out because of the baby. Many of times I cried and wept on DH saying I didn't know why we'd ever wanted to do it again. We'd ruined our family etc etc etc.
Anyway, the truth is I think somewhere in those first few months I slipped into PND. I did go to the doctor, I did tell people how bad I felt. I got help but it still wasn't easy. I'm not saying this is the case for you or that you're depressed but what I am saying is that going from 1 child to 2 children was by far harder than having just DS1 so I understand completely your feelings.
Please try and go easy on yourself. Take care of yourself and give your body time to heal and your hormones a chance to calm down. Try not to dwell on feelings of 'did we do the right thing in trying to have DC2?'.
Probably not what you want to hear but DS2 is 16 months old and STILL hard work and whingy. I often find myself counting down to his bedtime so I can have a few moments with DS1 without him climbing all over us and whining. However, I know it is temporary. I only have to look at DS1 now age 5 to realise that they grow up fine eventually! It will be worth it. You will be fine and you CAN do this.
If someone would have told me 16 months ago that in 16 months time I'd be sat having quiet evenings with DH to mumsnet with both boys asleep after a successful day with minimum tears from DS2 and lots of one on one time with DS1, I'd never have believed them. I really thought I'd never make it and I was clinging on by my fingernails.
Somehow things do get easier. Sometimes all of a sudden like people say after a few weeks. Sometimes so gradually that you almost don't notice it but slowly but surely, your family will get through this and you'll be stronger and have that other little person in your family to love.
Sorry for the essay!