How do you stop crying over little things?!?!?

I am 17 and I am getting so emotional! I can't stand it!! Today I got in trouble by my boss because I didn't arrive for a few weeks. I had an overabundance of schoolwork and I told him that I could not guarantee my presence for a few weeks, but noooooo...he is being intolerant. Then the supervisor had to come out and overhear the conversation. I starting crying when they called me out on this issue. It was SO embarring. I don't know why I cry over things like this, but I do. I got a B on a test and I just lost it. I start crying when things get hard, but I continue to push myself. I guess I see myself as kind of a perfectionist. I am starting to get frustrated with almost everything. It is so annoying. How do I toughen up?