I KNEW she'd have some reaction to him coming into our house, I just wasn't aware how it would put me over the edge. HOW DO YOU COPE with this toddler behavior? I guess I am not very go-with-the-flow.
I feel for you mama! I think the short answer is you just have to deal with it. I see your toddler has just turned 2. She is probably old enough to understand and be excited about the new baby (hence the singing and stuff), but not yet old enough to adjust her behavior for the new baby. My DD is 30 months and she still is 50/50 in terms of being able to be considerate for baby's naps, etc. Luckily for us my DD is super noise tolerant and we have a pretty big house. My solution is to put DS down with the white noise machine in his room with the door closed (we have a baby monitor), and play with DD downstairs. I try not to tell DD to live a hushed life since her life has already changed so much with the baby's arrival. I also always pre-warn her, "We're going to mommy's room and we have to whisper when we go upstairs now", rather than scold her after becomes loud. And always let her have an outlet. I see that your baby is only a few days old? You're probably exhausted which makes your noise tolerance low. Give it a week or two (or three or four) until things settle down. Also wear your newborn more for naps. Snuggling usually keeps the little ones asleep better.
My DD was about ten months older than yours when DS arrived, but she was also extra loud and exuberant when DS came home. Honestly, I ignored the noise level and DS quickly adjusted and can now sleep through most of her play. If she's being extra loud, I put him in the carrier and wear him while he sleeps. I never asked her to quiet down as long as she wasn't screaming or screeching. And I think she either quieted down after about three weeks or I adapted to it, because it doesn't seem as overwhelmingly loud around here as it did in those early weeks. (Except for her screaming when DS cries. Still working through that behavioral nightmare.) Anyway, I wanted to say that the balancing act of adjusting to meeting the needs of two young children and dealing with postpartum hormones was extremely challenging to me. And things did feel loud and chaotic ALL THE TIME, initially. (And I cried a lot more than after DD was born.) I don't think it has anything to do with you not being "go with the flow" enough.
I really felt that as I healed physically from pregnancy and birth, we began to find our way and I became much less frustrated by DD being a toddler (excepting the screaming when he cries.) It gets better.