T.S.
You definitely have a lot on your plate and it is no wonder that you feel like crying. Allow the tears to come. Do not resist them. It is your bodies way of supporting you in releasing some of the stress and anxiety you are feeling.
It is time to find some support. If you can get a second, third, or even fourth opinion for your daughter until you feel someone actually understands what is going on and can help her. You have the right to be mamma bear and demand care for her rather than giving your power away to doctors that act like they are gods rather than simply human beings with specialized information. Listen to your intuition. Your gut will tell you when you find someone that is really supporting your daughter's wellness.
You don't give any details about your sons life but I can't help but wonder if there has been a great deal of stress for him since he was really young. His symptoms sound more like him trying to cope with a stressful and painful life rather than there is just something wrong with him. Your son needs some support with better skills and tools to help him with his fears and stresses. He could really benefit from some play therapy and someone that understands emotional intelligence.
You could really use a counselor, coach, or group support. You need support as a mother and as a woman that is dealing with a great deal of emotion, anxiety, and overwhelm. If you are able to get yourself some support you will be better able to support your children. I know that most of us as women have been taught to help everyone else first and that we don't get to even be on the list becuase that would be considered selfish. That is a lie. The truth is that we cannot truly show up for anyone until we show up for ourselves. We must be on the top of our list or like you stated, we eventually just shut down. It is self-responsibility and self-love rather than selfishness.