Found a lump in my breast. So scared it's cancer

I'm 36, and have found a (little) fingernail size lump in my left upper left side breast, it's fully moveable I can sort of move it up and down and in circles if you get me? It's a little tender to move and press but not really painful. There feels like a hard thickness underneath the lump. 
 

i had a fibroadenoma about 8/9 years ago I was diagnosed at the breast clinic after an ultrasound. 
i can not remember what the lump felt like then. 
 

i am absolutely terrified it is cancer, I have 2 children I can not leave them I just can't. 
I'm going to ring doctors first thing. 
 

can anyone offer me any reassurance before I start writing letters for my kids for the next 10 years!