Why can't I stop my back from hurting after I deadlift?

I’ve tried everything. Immediately after deadlifting, my back is stiff and achey, and then it’s sore for anywhere from a day to a week. I have tried several times to correct my form, but at the very beginning of the lift, my lower back always curves slightly, and I think this is where the pain originates—this particular weakness in my form. Try as I might to keep my back straight, it always hunches against my will. What can I do to correct this?

Edit:

I would also love some suggestions about what to do if I can’t get it right. I can’t squat as low as I’d like and don’t always have a spotter or a rack, so my lower-body and lower-back lifting options get pretty limited.