Mizz - Sometimes the discontent comes out of nowhere & for no major reason. I usually am able to talk myself out of it by saying how overall lucky I am - but it doesn't always work. (I used to hate it when my mom would remind me that I had nothing to complain about - which invalidated my feelings.) As advbike said, it's a cycle we go through - the rhythm of life. I hope you'll feel more optimistic soon.
Let us know how the eye dr appointment goes.