I'm basically copying/pasting my response to another question from like 10 minutes ago. I can completely relate - especially about the sneezing part. I would have to "get in position" in order to sneeze and not have it feel like someone was stabbing a knife into my lower back. I was miserable.
I have a degenerative disc in my L5/S1 (low back) caused by my first pregnancy. Before this diagnosis, I saw about 4 different chiropractors over the course of about 16 months that all tried to fix me in different ways, but the nature of my pain/injury was not discovered until my OB recommended a real MD. I had an MRI and discovered that I had inflammation in my tissue that had spread to the bone, which apparently can be extremely painful (you're telling me...).
After getting an accurate diagnosis, I was able to effectively treat and overcome the pain. The first treatment was a steroid injection into my back, which didn't work. So I went on oral anti-inflammatory meds for a little while and was fanatical about getting strong - going to the gym every day at 5AM for strength training, doing Pilates a few times a week. It was a small sacrifice that worked WONDERS and almost completely eliminated my pain. I will never be 100% and had to give some things up like snowboarding and helping friends move (oh, darn), but I am so much better off now (even now and I'm 7 months pregnant and have a HUGE belly).
I'm not sure what core exercises you are doing, but sit-ups will most likely not help you at all. I recommend you find effective strength-building core exercises and be religious about performing them regularly. Good luck! I really know where you are coming from and it is truly miserable living with back pain.