I don't understand

I have my future almost perfectly planned out. my husband and I are going back to college. he to be a chef me to be a riding instructor. I am saving up to start riding lessons and pay off my loan so I can get financial aid. I am more excited about our future than ever. yet I still feel anxious and depressed. I have days where my hope and excitement for my future have me smiling and happier than ever. days where even the longest shift at work can't bring me down. yet I still have days where it's the opposite. where if my husband and I didn't absolutely need the n feel like a pathetic coward who can't do anything. I don't understand how I can feel so good one day and so awful the next. I just don't understand.