OP, I mean this very kindly- please do not waste energy and time worrying about this right now. I absolutely agree you should love your dress and maybe this one won't be right for you anymore, but you really don't need to decide at the moment.
You are only 12 weeks after giving birth. Your uterus has barely finished contracting to its previous size. Your rib cage and hips may not yet have started to come in again. Also, you clearly had a traumatic experience, and I doubt you've physically or mentally recovered and got to grips with life as a mum yet. Sleep deprivation is also a bitch for perspective.
FWIW, I wore a similar dress - long before children and I couldn't wear that shape now, as alas two sections and a lot of internal scarring have irrevocably changed the shape of my stomach. However, I had horrendous diastatis recti (7-8cm 😩) and my physio thought I'd need surgical intervention but I started post natal Pilates at about 16 weeks post partum and by the time DC2 was 8 months old I had no separation, so I hope maybe the same could work for you. It's also taken a lot of time (and a second pregnancy!) for me to understand and accept the time it takes our bodies to change and recover from pregnancy.
A brief note to Boggles: I am lucky enough to know women from many different countries and all appear to face similar issues regarding body recovery after pregnancy. Aside from the word "grunt" as an adjective being crude and puerile, the idea that all women should have great bodies after childbearing is unhelpful at best and potentially damaging at worst. My body will never go back after two sections, one to save mine and my baby's life. I was left with huge internal scarring, and I'm still struggling to lose the pregnancy weight after DC2 thanks to chronic back pain and depression. Or perhaps I just have low standards?