This is my third baby - but fourth pregnancy. It took me nearly 3 years to convince my husband to have a third child, and then when he finally came around, it just happened so fast. First he agreed to the baby, then he wanted to immediately TTC. I delayed it a bit so we could have an epic vacation without kids, so I could drink and enjoy the week. But then we started TTC immediately when we got home and I got pregnant on the 3rd cycle - when I expected it to take closer to a year, like the other 2 did.
In less than six months I went from 'I'm never having anymore kids' to "pregnant on purpose" and it REALLY took me a while to wrap my head around it. Then I was so sick with morning sickness, and hormonal and... yeah. I was not nearly as excited for this pregnancy as I wanted to be, and that made me feel so guilty.
I started coming around about 16 weeks or so, I'd say - truly getting excited about it all. Before that I could at least act like I was excited, but really I was just sick - and so over vomiting.
I don't know what caused the switch, but once I started feeling better in general, I felt more excited about the pregnancy. I was still a bit depressed for a few more weeks because my pelvis was giving me fits - but I'm seeing a physio now and I'm just thoroughly happy to be pregnant and SO excited to meet her.
Be gentle with yourself - its not bad to feel the way you feel, and you'll come around. Having older children does make pregnancy harder to cope with, and it definitely makes it harder to think about the pregnancy as toddlers are so demanding for attention.
It was much easier to be excited after the anatomy scan was over with and I could relax about the baby's health.