I can't even remember to put on my cute earrings.

OK, fingernail care is a particularly hard thing to do by discipline.  It is uninteresting once you've done it once or twice.  It requires frequent small applications of maintenance.  The rewards are pretty low, because a small thrill from seeing your pretty hands isn't all that compelling when stacked against stopping whatever better things you're doing to tend your nails.  No one else cares very much about your nails, so you aren't letting anyone down if you don't do it.  It is a task that can float, and those get pushed into the future.  Fingernail care may be easy if you like self-grooming, but if you aren't a person who likes that stuff, it is exactly the type of problem where self-discipline will fail.

I see two options.  First, you could combine nail care with something that is actually interesting.  Do you have a girlfriend who wants to come over once a week to watch What Not To Wear and drink a little something while y'all paint your nails?  It would have to be a very steady date if you want tended nails to last for months or years.  Second, you pay someone else to tend your nails.  Have a standing appointment to get a manicure every two weeks and always show up.  Going somewhere and being tended while you chat with people whose lives aren't like yours is a million times more interesting than filing nails.

Darlin', I'm not surprised that you've never kept a nail-care regime.  I've never tried.  But I think it is a hard thing to do for people who don't intrinsically like that stuff.  Moreover I think that being rough on yourself about it is the wrong approach.  It causes you unnecessary guilt, and more importantly, it doesn't work.  If you really want the problem solved, it is time to escalate.  Set up a system that gets it done.  Or, decide that compared with all the better things you could do with half an hour a week and your money, it isn't important enough to go to that kind of trouble and walk away guilt free.  But don't tell yourself that something so small should be easy and only inadequate you can't do it and you absolutely will this week or you'll punish yourself.  You're perfect, so make it revolve around how you are.