I understand how you feel to a certain degree. I have a two year old. I've been trying for another baby for a year on and off. It hasn't happened for so long I'd started getting used to the idea it was just going to be 1. Then BAM pregnant. We'd only had sex once that month and hasn't been 'trying' so whilst there was a chance I wasn't expected it.
I was initially happy. Then anxiety took over as I had a lot of bleeding. Then we finally got a scan with a heartbeat and the fear set in. What is it ruins my wonderful like with my current son? Can I really face those relentless baby days and not crumble and be a good mum for my toddler? What if he toddler hates the baby, or me? I've (non seriously) considered termination too. But the reality is I think these are normal concerns with having a second child.
I suffered terribly with pre and post natal anxiety and had post natal depression too. I was suicidal at my worst. I think that is playing a big role.
I finally plucked up the courage to talk to my husband about my feelings and he's been really supportive. We've talked about the practicalities and how we can work together to manage. What changes we can make and how we can prepare. It doesn't really matter if things work out slightly different because he's shown me there are as many solutions as there are problems.
I think a part of this is normal but you're feeling incredibly strongly both ways. You're hormones are all over the place. Whatever the situation you always have the right to chose but I'd really urge you to speak to someone, maybe even a counsellor and work through how you're feeling in a rational way.
I'm sorry you're having such a hard time. I'd always urge you to speak to your partner, if you're going to have a baby together you need to become a team that hardship will not break. Trust him with your feelings and see if he can help or just listen.