Welcome to WP! This site is chock full of incredible information so for starters I suggest you poke around a bunch. One of the very knowledgeable folks who posts here often, Tracker, has written a great book and posted it at http://www.asdstuff.com/grats.html
I think many of us suffer from anxiety that we are bad parents because what we are doing with our kids just doesn't seem to be working. At least we may feel this way until we learn that our kids need a different way of parenting that no one ever explained to us might be part of the package of being a parent. The great news is there are many resources for us to learn about ways that will work better. A friend of mine's child (4 yo boy) isn't diagnosed yet but is having some pronounced difficulties especially with self-confidence and anxiety. She tearfully confided in me that when she hears him say things like "I'm a loser, I'm no good, I can't do anything right" her heart just breaks and she fears that there is something she is doing that is causing him to feel this way. She has always tried to be supportive and has tried to build his confidence, not tear it down. I told her that her methods would work perfectly well with an NT child such as my older son but that its very likely that her son is on the spectrum and that his brain is probably wired a bit differently than many. The cause and effect cycle of what you as a parent say and do and what our children on the spectrum hear and perceive is completely different than it is with mostparents and NT kids. My friend is just starting down the road of having her son evaluated and is learning as much as she can about ways she can help her son. When I talked to her yesterday, I could tell she was already feeling better because she had caught a glimpse of a better road ahead.
I guess I relate this story because I know that it hurts to be a parent when you see that even though you have the best of intentions, your child is not thriving mentally, physically and/or emotionally. I think it is common for us to blame ourselves and feel very lost and afraid that other people are going to judge us harshly. In my experience, once I tossed out almost everything I thought I knew about parenting, things started to get better for my son and our whole family.