I was a mess for the first few weeks but I didn't have a slow, planned out, amicable split like you did. Mine was dramatic and happened really fast. I think it was more my body and mind coming to terms with reality that made those first few weeks so difficult. I'm much happier now, but I had to get past the drama to get here.
ETA: my baby also turned 4 months old a few days after I kicked RAH out and I felt so strongly like I had failed my kids by choosing such a crap partner to be their dad. The lack of sleep and hormones probably also played a huge role for me. I was such a hot mess. I think I was completely opposite from how you're handling your split. You're handling yours with so much grace and with such a level head. It's very admirable!