I think it's totally normal for us to sometimes feel like we aren't doing the best job. Trust me, A., you are doing a wonderful job and far from worthless! We have our up times and down times. It always helped me to have support from other moms going through the same thing. This board is a great way for that to happen, but having a person in the same room with you, to interact as adults may also help.
The local hospital in my area has a support group for mom's and their babies...that might be a great way to visit with other women...share/give advice on motherhood and have some adult interaction.
Try to schedule some time for yourself. I know it's very difficult when you are breastfeeding, but really try and make that happen. You don't even have to leave the house. Arrange to maybe sleep in an extra hour and take a long hot shower after you wake up with strict instructions not to be disturbed. Or go get your hair or nails done. Or, one of my personal favs, buy a magazine and sit in the backyard...alone! Ooooh - or make yourself a yummy breakfast and sit in front of the TV watching whatever cheesy show you have on DVR, undisturbed. Full tummy and cheesy TV!
Also, get out and take a brisk 30 minute walk everyday. You don't need a jogger...just a regular stroller. Endorphins make you happy!
You should also talk to your dr. They usually ask you in your follow-up appointments if you've got the blues, but you may want to schedule an appointment.
And know this...you are a wonderful mom with great purpose!
Hang in there!