Then make it the width of the container. (a.k.a 1200px)
.sticky {
position: fixed;
top: 0;
left:50%;
margin: 0 0 0 -600px;
width: 1200px;
}
And when you reach 1200px use media queries:
@media screen and (max-width: 1200px){
.sticky {
left:0;
margin: 0;
width: 100%;
}
}
That's all.