I am so glad you have somebody as lovely as TelephoneTree to hold your hand here 
Often encountering an opinion that is different to the one in your gut is what helps make you realise what your gut feeling actually is, whichever choice it is. So in many ways your mum has unconsciously done you a favour. I would feel like you if that was suggested to me, in fact that is why we waited it out as we knew we'd keep our baby regardless. I honestly mean no disrespect to others who have chosen differently - everybody has their own hard decisions to make.
When we were waiting it out I became a lot more conscious of people (adults and children) with Down Syndrome in the wider community. I had never really paid attention before, but I do now and I realised I had a lot of misconceptions before. Just last week we were at our local snowdome and there were a few youngsters with Down Syndrome skiing to a great standard. It's not the closed door so many expect it to be, so long as the health issues aren't totally prohibitive.
I am obviously not able to speak as someone who has had a positive diagnosis and has been through it, but I would imagine it is safe to say that it is okay to be sad and grieve the loss of the future you were expecting. If you haven't come across it before, the "Welcome to Holland" story rings a lot of bells with my friends when they have had their children newly diagnosed with special needs.
Very much thinking of you and your emotional journey ahead.
By the way - give them a call tomorrow and ask the sex. Pretty sure they'll have that information somewhere - they test every chromosome after all. And then let us know! 