First off let me tell you something that I am sure you can never hear enough; you are a great mother. It is never easy to deal with disability in your child and while I cannot speak to personally being a mother of a child with a disability, I have worked with children with disabilities, specifically one family and it was one of the greatest thing that has ever happened to me. And I know it is the best thing that has ever happened to them.
I know that seems counter-intuitive, but I have seen it in action. It has not been a perfect journey for them, nor a painless one, but today these are some of the happiest most well-adjusted people I have ever met. They employed several techniques, most importantly that of the Option Process. I highly recommend the book, Happiness is a Choice by Barry Neil Kaufman. It was life changing for me and working with "my Angel" (as I call him,) was truly trans-formative.
Please don't take this as being preachy or me saying that there is anything wrong with they way you feel now. It is totally your right to feel however you want and all of your emotions in this matter are valid. But I have seen that there can be light at the end of the tunnel and moreover, I have seen how what one person can consider a tragedy one day can become the best and biggest gift the next day. My favorite saying I took from the process was, "The cause is in the future." This means that while you may not understand why this is happening to you at this moment, there will come a time, maybe tomorrow, maybe 30 years from now, when you will look back and be like, "Ooooh... I get it now."
Lastly, this may not be a belief you share, but I have come to believe that there are reasons we are given challenges in life. It is not some karmic punishment but instead a chance for beauty and love to grow and flourish. I also believe that our children choose us in ways we can't even begin to understand, (a soul-contract, if you will,) and that if this beautiful little soul chose you, you must be a very good person ready to learn some very big cosmic lessons.
I know this may not be the answer you were looking for, and again, please do not think I am sitting back and saying, "just deal with it." I can't possibly understand how hard this must be for you. Just please know that I am sending you love and giant hugs and the wish that this process be filled with as many (or more!) blessings as there are hardships.
Hugs!