the last straw

or: the last straw which breaks the camel's back
meaning:
a small addition to a burden which causes it to exceed the capacity. 

source: Red Planet Cartoons

 examples:

• Claire had a bad day in school yesterday. She lost her favourite book, then she fell down, and when she realized she had broken her glasses, that was the last straw and she began to cry.
• For me, this was the last straw.
• The last straw came when an old man stared at me and I could see in his eyes he was undressing me.