My Weakness Is That I Care Too Much

I don't even know where to start.
I would say that my sensitivity started after a traumatic time in my life,  so I figured that I had to keep my emotions all locked up inside. Everything that I felt just kept building up and breaking me down. I soon felt that I was the only person who would feel a certain way about things, and I would hide my emotions out of fear of what others would think.  So I had now had the idea that I should not show emotion, along with self confidence issues. I still happen to hide my emotions to this day, out of habit, and the fact that I feel vulnerable when people see me crying. I don't take constructive criticism well, because it feels like people are attacking me and it points out my flaws and makes me hate myself and feel useless.
Anything can set me off, whether it's something that reminds me of my past, a touchy subject, or something that generally makes me slightly unhappy, I START CRYING. Once I start, I can't stop, it's almost impossible. I get really frustrated easily, and when I do, I cry then too. It's like I can cry at the drop of a hat. It's really annoying and something that I wish wouldn't happen. I guess it doesn't help when my sensitivity is coupled with hormones.
I'm very empathetic and sympathetic towards people, and because of this, I want to help them. I usually worry about them too much and end up getting hurt. I can also pick up on peoples' emotions and tell what they're feeling, and take on that emotion. As hard as I try, I can't stop myself from feeling other people's emotions. That's why I dread going to school everyday, everyone is so negative and it brings me down. My friends thinks that I am a strong, happy person. Wait until they finds out that this their friend is a sappy wimp that cries at everything.
I obviously care too much, otherwise, my sensitivity wouldn't bother me. My sensitivity is a weakness which makes me feel vulnerable.