I'm quite upset and angry on your behalf here. I shall try not to diss your ex-MIL, but I don't understand how anyone could be so horrible. It's going to be very hard for you to ignore all this and be happy anyway, but do try to! Don't let them take one bit of your happiness or confidence away from you!
I'm 34. I have a form of bi-polar illness. I have two kids and am trying for a third (an evap line on a BFP got my hopes up and brought me prematurely to this forum, now I've got hooked, following people's stories etc I'm a sort of gatecrasher but hoping to be here legitimately soon! LOL) I know there will be a couple of people who think this decision is wrong, but as I see it, my daughters enjoy being alive and are glad to have been born. Same goes for me right now; though there are times when I wish to die, the good and okay times make the balance in favour of life overall. I think life is the most precious gift, and things have to be very very bad indeed for someone to feel it would be better if they had never been born.
Are things going to be that bad for your baby?? No way!! This baby has a home and a mother who will love him or her with all her heart. Your baby is going to be very glad to have been born. No one else's opinion matters. Screw them!
Things might be tough for you and you are going to need a lot of support, reassurance, and love. Spend as much time as you can with the people who make you feel confident and happy, and try as much as possible to avoid anyone who undermines that.
I think, if I were you, I would say to the naysayers "I am having this baby. That is my choice and I have made it and there is nothing you could say or do that would make me change my mind about this. This baby is a person [and your child/grandchild/insert whatever applies to the person you're talking to!] and will be loved and he or she deserves to feel welcome in the world. If you really care, then the best thing you can do to make this situation better is help me to celebrate and to feel confident and capable. If you aren't willing to do that, then I'm sorry but I don't think we should remain in contact until you have a change of heart." It might sound harsh, but you will need to be tough, even ruthless, for the sake of your sanity and well-being and that of your baby. Don't let anyone drag you down or undermine your confidence and your happiness.
As for having wanted a baby, but not "like this". Life happens. People can fall pregnant in ideal situations that then go horribly wrong later on. At least you know what you're letting yourself in for, you know? You can be prepared for the likely challenges because you know what they are likely to be.
You need to believe that you will cope. I know how hard it is to believe in ourselves when we have suffered with mental health problems, but I also know how important it is. You can cope and you will cope, and next Christmas you will have the magic of sharing it with your child, and you will look at your child's face and know without a trace of doubt that you made the right decision. Be happy while you can. Enjoy every minute you can. You're having a baby!! Yay!! :-)