I really struggle with my first child, just because she had some undiagnosed health issues and was just an incredibly demanding baby (even her childminder said she adored her but found her exceptionally hard work), so the idea of another was the last thing on my mind, until my partner convinced me when she was 18 months old, things were slightly easier, and more importantly, everyone told me that second babies were always easier.
So I decided to go for it, almost principally because I didn't want DD to be an only child. The first 10 days were fine and then colic got him too as it had DD. It was hell and I do remember sobbing my heart out and feeling an overwhelming thought that I wished he'd never been born, grieving the fact that we had finally found some level of stability with DD and that we were right back to where I was.
Despite these feelings, I did feel a strong love for him and I guess that's what got me through it. I took it day by day and waited for the time it would get better. It took 5 months.
He is now 13yo and I can't imagine my life without him. The thought that I once actually felt regret at giving birth to him seems surreal. Yes, my recollection of this time is fretted with stress, not the great memories that most mums seem to have, but I certainly know I did the right thing.
As newjob said, only you can make that decision though.