Oh, sweetie (((hugs))). This is totally normal to feel guilty about the work and the breastfeeding. I'm a big believer in breastfeeding whenever you can, but honestly, a happy momma is more important. If the struggle is too much, then baby will be better off with formula. Actually SEEING your children is more importan than pumping at work and being late.
You could try to just nurse at night for as long your milk holds out if you want to. Be prepared that it might not last for long, so that you aren't going through this all over again. But every little bit helps and maybe it would help you make the transition. That could be a good compromise IF it doesn't cut into your whole family time too much.
Yes, you will always miss being with them during the day, but take pride in the fact that you are making a sacrifice for the well-being of the children and family, and they are being taken care of very well. You are being the best mom you can be!
If you still have the feelings of weepiness after you decide what to do about the nursing and then make that transition, you should probably check in with your OB to make sure it isn't post partum depression. It will be normal for you to grieve the end (or compromise) of breastfeeding, but then you should start feeling better once you let yourself move on.
Good luck, and I hope you feel better soon!