Sounds like someone doesn’t want you to be happy.
I’m thinking that this kind of vileness says alot more about the person that sent it than it ever says about the person they’ve decided to fixate on. It absolutely isn’t about you, it’s about them.
From what you mention, it sounds like someone has had their nose put out of joint by you perhaps moving on in some way? Moving on to new possibilities, being more confident and sparking interest in a certain maaaaan (!). In fact, this date seems to be a trigger, perhaps someone had a thing for this guy, or a thing for you... or simply a vested interest in you being always single, or ‘not on the market’ for some reason.
Anyway, it will be about them and about how their insecurities have been roused into freakishly raging anger and for some probably irrational reason, they’ve fixated on a situation that you are part of as the cause of all their woes, and they want to make you, a completely innocent and unknowing person, feel bad, so they feel better. It’s revolting and twisted, whoever it is must be such a small and powerless person to get their kicks doing this.
You probably aren’t even the first person they’ve done it to!
So I wouldn’t spend any more time or ‘share of mind’ dwelling on yourself and worrying what about you is so awful to prompt this kind of message.
It’s really not about you so you won’t find answers looking inwards.
Please don’t tear yourself apart trying to find the reason, as there just won’t be one and you’ll make yourself doubt everything about yourself.
And you will of course be a mix of good and wonderful and flawed and human 
... because you are, err, a human, I hope anyway, unless mumsmet has become popular with a Martian audience as well!
See, bad jokes are one of my hideous flaws, and not the worst either, this letter writer would have a field day on me!
As an aside, it sounds like you are an amazing mum who has a wonderful bond with their child under very difficult circumstances. I bet the nurses know that at the hospital/s where they must know your faves all too well. I bet they see the love and sacrifice you give willingly every day your darling child has to be there. And i bet the cleaners and health care assistants see you too. I bet the cashier in the local coffee shop near the hospital sees it. And anyone sitting at the bench outside the hospital grounds too. I bet they see the struggles and the pain and the hardship you yourself go through as you do everything to take on your child’s pain and suffering in every way you possibly can. Anyone from that part of yours and dd’s life will see what’s plainly there (... and what I can feel coming through strongly in your posts!).
I don’t know what family or close friends they’ve got, but also, I suspect that anyone who has the privaledge to see the little everyday glances and touches between you and your dd will also be able to see truly the love that’s there. It will be in the little things, not the flagship ‘making memories’ Facebook fodder. It’s subtle, like when she reaches back for you without looking round because she senses you there, or maybe the opposite, it could be when she launches away so confident because she knows her mummy’s arms will always be there to run back into.
I bet anyone with eyes to see will have seen the care and love you have for your dd.
But to see such love you do have to have the eyes to see it and the mind and heart to recognize it... there’s that saying isn’t there, there’s non so blond as those who will not see? Well, some people won’t see what’s there for their own reasons. Again, it’s not to do with what’s there to be seen. It’s to do with the person doing the seeing.