I love your colors and the mat. I know that the circle binding can be a pain. The thing is if you had cut out a larger circle of the light green and appliqued the circle onto it that would have been easier than trying to ease in the fullness of a wide strip.
When you are going to put a binding on a circle you MUST use bias binding, press it only on one side to stretch it slightly on the outside and take a runing stitch around on the inside to take up the slack, pulling ever so slightly to make it the size of the round of the hem, pining in place as you go, then snip tiny snips on the inside to take out the fullness (or bulk) . That side gets sewn to the topper. Then when you turn it to the front, the outside is stretched to fit better. That would help tons with this design. But, like you say, it is a pain either way.