Nursing is making me really upset!

I have been nursing for over 3 years. I thought I had encountered every problem but it seems not. My older daughters nursing has been bothering me for a few months because she doesn't seem to know how to latch properly anymore. We have tried many times to fix it but it doesn't help and she is not ready to wean so I just deal with it. Now the baby's nursing is driving me crazy too. It honestly feels like she is flicking my nipple with her tongue rather than nursing properly. I started giving her the occasional bottle about4 weeks ago and she didn't seem to have a problem. I think she's only had about 6 bottles. Now I am wondering if that has messed up how she sucks. She latches on over and over and over again with this little teeny mouth and I can't get her to open it wider. If I do manage to get her to open it wider she just slides is small again a few seconds later. The resulting sensation of nursing makes me want to vomit. It is so totally horrible to me, it makes me want to crawl out of my skin. I have to sit there digging my nails into my skin just to get through it. It doesn't hurt at all it just feels gross. I have to use a lact-aid with every feed because of a breast reduction so that creates a problem too because it is hard working on the latch and getting the tube right plus even if she does the bad latch she still gets milk via the lact-aid tube. I am almost positive my milk supply is going even lower because of this because her poops have been looking more formula poops rather than breastmilk poops. This is bothering me to the point that I feel like just weaning both girls and telling everyone to stay the hell away from my breasts! Help!