I think you sound like a wonderful mother! I think you've recognized a very important thing, which is that he needs help learning to regulate his emotions better. You've decided to have him evaluated, which can be very helpful and is not an easy decision to make. He's doing the best he can, and so are you.
It's okay to be sad and frustrated. And yes, sometimes if they sense a lot of that it can make things worse. One thing that helped me was to set aside private time to be sad, to grieve (because this life has turned out to be so different from how I imagined parenting would be, and it's okay to grieve the loss of that dream/expectation), to be frustrated. It helps to talk with supportive listeners. It helps to make sure I get some time to myself, and take care of myself. Parenting a challenging child requires a lot of energy and endurance, and I have to take care of myself in order to be able to care for my child. I know that if I don't take care of myself, and I get to the point where I am just completely drained emotionally, I cannot parent my child the way she needs to be parented. I'm going to reach that point sometimes, because I'm human and because sometimes circumstances prevent me from recharging as much as I need, so I don't beat myself up about it but pick myself back up and go on doing the best I can.
While you are waiting for an evaluation, I recommend that you read The Explosive Child: A New Approach to Understanding and Parenting Easily Frustrated, Chronically Inflexible Children
by Ross Greene (the newest edition, 2005 IIRC, is the best one to read). www.thinkkids.org
You aren't a failure. You are a loving, human mother doing the best she can under some difficult circumstances.