How do you keep a check on your expectations?

I have a certain friend whom I love and I've told them this. They don't feel the same way about me though. I've tried to stop talking to them but they really want to stay in touch and maybe it would have been an okay arrangement but the problem is- everytime they start acting all sweet and nice I start imagining all those scenarios of us being together forever and blah blah. I know it is stupid and I know that I should learn my lessons by now but what do I do with this blind, ignorant optimism? I'm really in pain...