Hope... An optimist's tool to get through each day?

I've seen myself as an optimist. I haven't always been one though, but I realised that having a positive outlook  makes it easier to get through each day. Most of that positive thinking is concentrated on 'hope'. Hope for the better, hope for the future, hope for everything. But is it all an illusion? Is it just our mind playing tricks on us, to get us to see a blurred reality?
Hope may be a tool for us optimists, but how far does it get us through life? For example, if I were to keep hoping that dodo birds are going to be around again, that's a little too far (I would have said dinosaurs, but all those that believe that Jurassic Park can happen would throw eggs at me :P ). But, if I hope that somewhere along the line, they find a drug that can target a specific set of mutated genes, or fix a mutated codon, it may be a little more realistic, but not entirely, because who knows how long that will take.
Where is the line between being an optimist, a realist and a pessimist? Is is how much we hope in our day to day life? Or is it how much we believe in the betterment of tomorrow, that things will get better than they are now, and that there is some good reason for our bad misfortune?
Is hope seeing the light at the end of the tunnel, but not knowing that the light is from an approaching train?