So sorry to hear this @mrsr87. I think you are perfectly entitled to feel this way, it is seriously shit.
I have also spent the day in floods of tears. My DH was made redundant in the summer and the tension in my house has been unbearable. He is trying his hardest and has had quite a lot of interviews but each knock back just sets us back. I am sick of being in the house and sick of being in bed all the time (the HG is still pretty awful). It just feels like such a sad house, when it should be such a happy time.
If you asked your mum to isolate beforehand, or pay for a private test, would you be happy breaking the rules and having her over anyway? I certainly wouldnt judge you for it. I think your mental health is important, espcially post natally.
Is there something you can do to cheer yourself up today? Eat something nice, read a trashy mag, take a nice hot bath? Allow yourself the time to acknowledge this is proper shit.
I myself am a Christmas nut so am going to drag myself out of bed, put on a festive film, eat a mince pie and start writing out Christmas cards... I hope you manage to feel a bit better.