Hello,
I was not even a member of this site but joined to answer your post. I have been through exactly this - left for someone else while 5 months pregnant. I felt suicidal, lost weight, had to take antidepressants, dropped out of university, and all round had a miserable time. I just want to give you some information on my experience that might possible help you.
Do not let him make decisions about whether he will be present, what involvement he or other people have. He has left you on your own, and only an idiot would not understand how devastating that is for you. Don't beg him, or try to find out more information on what led him to leave you, the fact is it doesnt matter why. When you feel suicidal, try to think about how you will feel when your child is here. Regardless of how hurt you are, how hard it will be to be a single parent (there's no denying it), your child will be the best thing in your life and the absolute greatest source of happiness. Just looking at them will make you realise that all the pain you're currently experiencing was worth it.
I personally had antidepressants to carry me through the suicidal thoughts, and also some counselling. The counselling did me a lot of good and I think because you also sound very alone it would benefit you too to have someone impartial to talk to.
Right now you undoubtedly feel like your life is awful and unbearable. What you can't see yet is going through something like this will completely change who you are. You will be strengthened and more wise, and in the future you will recognise assholes like him more easily. You as a person will be better from this experience, with the added benefit that that absolutely horrible selfish spineless (etc) man is no longer sapping your energy.
What you probably need from him is space - and a lot of it. I personally cried for my ex at the birth (he didnt care about being there) but that was because I was still in love with him. It is of course good for your child to have both parents involved, but make your decisions based on what is best for YOU, because ultimately what is best for you is best for you child. Allowing your ex to treat you like a doormat is not a good example for you child, and it is not going to help rebuild your confidence. I completely humiliated myself in addtion to being humiliated by him, by begging him and trying to cause trouble in his new relationship. I know how desperate you feel.
The most important thing is to rally support for yourself from anywhere you can get it - GPs, counsellors, midwives, your friends. Tell your friends what a bad way you are in and ask them to help you.
I really hope that even one thing I have said gives you some insight into the future or makes you feel stronger or better. I personally benefited hugely from my experience, I learned more from it than anything else in my life. I'm sure that in time you will experience the same, and although it won't be tomorrow, next week or even next month, you WILL feel happy and strong again, you will have a piece of crap that was weighing you down out of your life, and your child will be there to bring happiness into every one of your days.
Please contact me if you need anything, I will do anything I can to help you in any way.
Alice