Oh @LlamaGiles. I'm so, so sorry.
My beloved DH died suddenly almost a year ago - my daughter was 2 at the time.
These early days are unbearably painful. Even remembering them makes me feel sick. There is nothing anyone can say that will make it any easier at this time - I really wish there was. But please know that it DOES get easier. It really does. You won't be in this agony forever. Right now all you can do is take it minute by minute. Don't think ahead. Concentrate on doing what you can with your daughter, and otherwise just making sure you eat and sleep when you can. I drank sweet tea because I couldn't face food. Please also see your GP - you may need some medication to get you over these first painful days and weeks. Diazepam helped me and I was able to stop taking them quite easily once I was ready.
Please also post here as much as you need to. It helped me so much to have a safe space, and to know I would wake up to supportive messages.
When you feel ready, do consider joining us at Widowed and Young (it is for anyone who has lost a life partner under the age of 51, you don't have to have been married). The community of people who understand has been invaluable to me.
I didn't believe people in the early days when they said it would get easier. I thought they couldn't possibly have loved their partners as much as I loved DH because I couldn't go on without him. But it's true. You get better at carrying it. The immediate agony eases. The waves of grief get further apart, and you learn how to handle them. I am 11 months on and most of the time I'm "okay", and I even experience moments of joy again. You will too, I promise. Not yet, but you will.
And please know that your DD will be okay. I got great advice from Winston's Wish on how best to explain things to DD and it was really helpful. Unfortunately it's a case of explaining it repeatedly which can be hard, but it is worth it as their understanding grows. DD talks about DH a lot now and obviously misses him, but she is in general a very happy, well-adjusted little girl. Also she will get used to seeing you cry and it won't upset her - it's actually very important that they see you expressing your grief and understand why.
I am so sorry this has happened to you. It's desperately unfair and there is nothing I can do to make it easier just now. But please know it WILL get better. It won't go away, but it will get better. Sending you love and strength. PM me any time.