I think you've already said what you needed to say...that you've "tried to train her". It seems that both you and she need a better teacher. The behavior you describe is that of a dog who seems starved for attention and good training. Leaving her in the garage "for long periods of time" is only going to make the situation worse for you both. I am not a dog trainer, but I do have three dogs and I know how much responsibility taking care of a dog requires. We do have members on this forum who are dog trainers and perhaps they can give you some specific information. You definately need to make a renewed committment to Darcy's care and training. I do sympathize with what you are going through, I've had some frustrating moments with my guys too. You began by telling us how much you love her and I think that shows wonderful potential for a really great relationship...you just need some better help with learning to train her.
Again, I'm sure some trainers on the site can provide some advice and/or resources for you. Hang in there.