Oh baby blues that's awful - I really feel for you.
This happend to one of my best friends who's husband left her for his first girlfriend from school after chatting online - she was about 14-15 weeks pregnant at the time and initially found it really hard - but i said to her at the time, if someone is capable of being that cruel - you are better off without him and although it's horrible for you now, and very upsetting - try to rise above it and show him you're going to be a fabulous Mummy.
Her son is now 3 and she has a lovely new partner but she had some tough times before she got there.
It's good you have a supportive family round you, and surrounding yourself with positive people will make you feel better.
Have a look online or ask your GP as I know in our area there are groups for single parents both for before and after baby comes and that way you'll have some good support mechanisms in place and be in touch with people who understand how you are feeling too and can give you advice.
As for the other girl starring and smirking - I would say just be really friendly even if you feel ready to explode - if you are pleasant you're not only showing you're the better person but it will make her think.
I also think if someone leaves someone to be with someone else - like in his case, he is unlikely to stay with her - chances are he will leave her for someone else too so karma will come into play, maybe not this week/ month/ year but it will do eventually.
I would also get to citizens advice or take a free hour at the solicitors - my friend got £50 per month when she was pregnant from her ex for maternity expenses (clothing, healthy food) and a one off £500 for baby stuff he agreed to give her as voucher form and now she gets proper maintenance. By doing this, you're showing him you mean business and you're not to be messed with.
I hope it all goes ok for you - Bella xx