I know how you feel. I had PPD after I gave birth, and that rolled over into full blown depression until my son was about four months old. I never really had anyone confirm this (I mean a doctor) -- but I knew something was not right, and it wasn't. Even though I relayed my symptoms to my doctors, I was too embarrassed to tell them the whole story and they never really asked, and besides it was all so awkward. You never really have time to talk to your doctor anyway. Finally, a little thing about me -- I am a doctor myself, and let me tell you -- most docs are not well trained at really recognizing signs of PPD. Just a few other pointers:
In terms of bloodwork, did anyone check your thyroid function tests? I don't just mean the usual TSH, T4, T3. What about thyroglobulin antibotidies? Did anyone check your thyroid during your exam to ensure it was not enlarged? If you don't trust your doctor to be complete, you may want to ask to see an endocrinologist (hormonal specialist.
Are you breastfeeding? There is a phenomenon in which mother's report feeling depressed while breastfeeding (strange but true) -- and it would be a stretch since yours is 2 years old, but still, thinking of all possibilities.
Has your child been evaluated for her "services" -- I am not sure exactly what you mean by this, but thinkg from a doc's perspective, she may benefit from a complete evaluation from a behavioral specialist to see if there is an underlying disorder that can be fixed with more specific therapy, or if there are discipline issues that can be better addressed by both of you going to therapy together.
Speaking of therapy, have you considered therapy for yourself? I have had bouts of sadness throughout my life and I have found therapy extremely helpful. Some may not mind having your child in the room with you, and some may be associated with a church or nearby care center where you can drop her off for an hour or so. It may take a few visits with a few therapists to get one that clicks with you. If you are in the Jackson, NJ area, I can refer someone I used that really helped me.
Finally, as all of us responders will say, I am sure, YOU ARE NOT ALONE. This happens to many mothers, and just because you are a mother does not mean you have superpowers (well, sometimes you might!). As one responder said, it may be a matter of lowering your expectations -- not for your child, but for yourself! It is OK to have a crying and whining child sometimes (well, a lot of the time, I dealt with my own little monster this morning). IT WILL GET BETTER, but please, keep asking for help, and don't allow yourself to suffer alone. You don't deserve that. In the meantime, good luck, and I will be praying for you and hoping things get better fast.