Hi Violet! So sorry you're feeling depressed over your coming GC (grandchild), instead of happy! I'm not sure if you know whether or not DS's XGF (ex-girlfriend) plans to have and keep the baby or whether or not you want to/think you'll be part of that baby's life or not. So, like MaMa, I'm not entirely sure where your depression is coming from. But since you said your DS "accepts his responsiblitity," I take it that, as far as you know,XGF is keeping the baby and DS plans to pay child support and, perhaps, spend time with baby, as well.
If that's the case, then I agree with most of the advice PPs (previous posters) have given you, though. Also, trust me, you are going to love this baby - and so will DS - even if you don't think so now. Perhaps realizing that will help you get past your sorrow and begin to accept the situation and even get excited about it, as Nana mentions, above. I remember being upset when my YDD (younger dear daughter) was pregnant and single with her first child. But thinking about how much I knew I would adore that baby once here helped me to begin turning my feelings around. I know you feel for DS, too, of course, But please consider that, someday, he may cherish this child's part in his life and be amazed that he was ever "torn up about" their arrival.
But most of what I said about you depends, largely, on whether or not if you're going to have a role in baby's life. And I agree with PPs that, if you want that or think you might, in time, you must avoid letting DS and/or his XGF (ex-girlfriend) know that you're upset. It can be very hard for people to "forgive and forget" the fact someone was against/unhappy about the birth of their baby. As PPs have said, vent here, any time, and/or to a therapist, etc - but never to DS and XGF! (If you already have, please, if you can bring yourself to do it, tell him/her/them that you're sorry and that it was just a momentary reaction).
I know you may not be able to see it now, but, in the end, this, most likely, will be a "joyous occasion," regardless of the relationship between the adults! This is DS' 1st child and your 1st grandchild! So I'm joining those who said - Congratulations!!!