Good days and bad days

When I have a bad day (like yesterday), I feel like I'm always sick and I'll never be able to do anything. When I have a good day (like today), I feel like it's good to get stuff done, so I should do things every day. As if just deciding to do things were all it would take. Even though I know the difference is that some days I'm sick and some days I'm not, somehow whichever way I feel, my mind seems to think that is the only way I will ever feel.