Hi mama, I know how you feel! I'm experiencing a similar weaning, for the 2nd time now.
In my last pregnancy my 17 month-old weaned when I was about 3 months pregnant. I had hoped to go longer with her, but it was such a natural, easy weaning there was no way I was going to insist on her still nursing.
Now it's around the same time, and my 20month old is weaning. He will still occaisionally ask to nurse, but I ask him if there's any milk and he shakes his head, and I never feel a letdown.
Last time I wasn't ready, I felt sad and felt like I had to mourn for our changing relationship, and I wanted to talk about it. I had read La Leche League's 'Mothering your Nursing Toddler' and it has a chapter about weaning with pregnancy, and it was comforting that while LLL fully supports tandem nursing, it is nearly universal for children to wean during pregnancy.
Do what you can to express your feelings. I found it helped to focus on other areas that our relationship was developing, and reminded myself to take the time to focus on your toddler like you would have if you were nursing. In my experience, my milk supply dropped and I didn't have any issues with fullness, at all. I also tried to focus on how straightforward, easy and natural the weaning felt, and I reassured myself that my baby(ies) was obviously ready as they adjusted so well.
It's tough when we're not ready! It feels like we're losing something special we had with them. I hope you find some peace in this transition and also pride in yourself and your baby in reaching this milestone