Girl at work makes me feel depressed

I have recently started to feel depressed at work but only when a certain girl is in. I used to have a crush on her which tore me to pieces. I was able to get over this or i thought i did. I'm unsure whether i actually have fully got over this. Everything seemed ok i would enjoy her company rather than have the crush feelings getting in the way, but i think its starting again. I dont get the butterflies and all of that stuff but i feel down in work and i barely speak to her or even look at her. I assume its a case of being aroung that which you cannot have. Anyone have any views ont his?, thanks