The world we live in is full of Dementors. There's always some thing or someone to bring you down; make your spirits low. Even if you think you're in your best mood, a Dementor will swoop in and suck away every happy feeling you had.
All you need is to work on getting your Patronus out. Hold on to something that matters, that keeps you happy. Never let any Dementor come near you.
Don't feel low if you can't work out on the Patronus at the first, it's hard. But trust me, once you get a hold of it, even your worst fear can't make your insides empty.