Hi there LaTristesse, I am sorry to hear that you are in so much pain
. I did find the first trimester really difficult in terms of nipple sensitivity AND exhaustion. How often is your DS still feeding?
I did (and have) found myself using various strategies to minimise or shorten feeds as much as possible (DS was still feeding through the night until mid June - about 20 weeks - which was the worst of it). I offered a lot more cow's milk in a cup/favourite snacks instead of a feed and if he was/is absolutely hell bent on having a bf, I would let him feed until it was getting too much, and then tell him I would give him a count down to ten after which he had to come off. That seemed/seems to work quite well.
The nipple pain (for me) eased off almost completely in the second trimester and it seems as if it what does happen usually. 
Unfortunately DS's writhing has become worse, which I put down to him struggling to get any sort of volume of milk, so I am hoping that will change when DD is born, but I actually haven't found it to be a problem. I am rather large now (almost 31 weeks) and we just work around the bump.
The emotional ambivalence has caught me off-guard. I know is normal, but I do go through periods where I just can't stand home feeding at ALL and others where he morphs into my sweet little baby boy 
Also, have you read the sample chapter and some of the articles on the Kellymom, tandem link? They are mostly written by Hillary Fowlers (I think) and can be very encouraging.
We are here to listen to your frustrations and commiserate too, but I just want to add that if you feel unable to cope or that you feel that nursing while pregnant is damaging your relationship with DS, there is no shame in weaning him or cutting down on his feeds.
Come and chat and moan here, we know what you are going through.