I don't know about medication or anything for this but wanted to offer words of encouragement because I felt exactly the same when I found out I was pregnant with DC2 - didn't feel excited, just sad about the fact that DC1 would no longer be my sole focus, not bothered about telling anyone I was pg, etc etc. I was worried about how I would find enough love for 2. I was worried I wouldn't love DC2 as much as DC1. I just managed to console myself in thinking that I always wanted DC1 to have a sibling, and that was the reason we were having DC2 (don't flame me, DC2 was always planned and wanted 100%, but pregnancy hormones made me need to rationalise it in different ways).
Anyway, as I progressed through the pregnancy, I gradually became used to the idea, but right up to the end, I was dreading going back to the baby stage and mourning my DC1's loss ofbeing the baby.
But seeing the huge smile on DC1's face when she saw DC2 for the first time, and seeing how much love they shares, DC2's HUGE smiles for DC1, makes it all worth the worry. As soon as labour started, my bond with DC2 was solidified. It was our journey, just the 2 of us. And when he was born, I just couldn't have loved him more. And someone said to me when I was in the same black hole that you are in, when you have two, the love doubles. And it's true. I thought I would struggle to find enough love for 2, but actually I have more love now than ever before, and DC1 doesn't see the 'loss' that you are mourning on their behalf - they just see that they have a fab new baby to play with and love, and that really helped me. She was happy, so I was happy.
Sorry for rambling, sleep-deprived, but wanted to let you know that what you are feeling is, in my exp, normal and for me it passed slowly through pregnancy and completely once my baby made his entrance. I'm sure the same will be true for you. Try to find some time each day to reflect and 'talk' just to your new baby. Sounds daft, but stroke your growing bump, talk to it in your head, develop a special relationship just the 2 of your. Then when your older DC is around, enjoy THAT special relationship.
I also read one thing that helped me realise there are ways to cope - it said 'enjoy your new baby in the middle of the night when you are feeding them, and give the rest of your attention to your older child'. Sounds a bit extreme I know, but it helped me to realise that I could prioritise my DC1 even with DC2 around. DC2 initially needed milk, warmth and sleep, with cuddles happening by default when feeding. He didn't need conversation, entertainment, etc. So I saved that energy for DC1 . THat is how I split myself down the middle to start with.
But soon the love just grows and you find your way and you all settle into place. And you will love seeing your DC1 with your new baby. ANd trust me, second babies are soooo much easier, because you know what you are doing and you know there is light at the end of the tunnel. The time flies by, so try to enjoy it.
Hope that helps somewhat! Sorry for rambling. 