Whether your glass is half empty or half full, just think, "I just saved somebody from dehydration." lol
I know that's a funny way to look at it, but that's life. A missed opportunity for you could've potentially saved someone's LIFE. You're probably thinking, "but my life needs to be saved too." Opportunities come and go. The ones that pass you by make room for better opportunities. Instead of thinking, "I just missed out on a nice opportunity," you should rather be thinking, "I just made more room for a GREAT opportunity."
Some think the optimism is an idealistic or unrealistic approach toward life. Contrarily, I think of it as hope--something we need more of these days. In a world that is lacking love, joy, and peace, why not be optimistic? It's kind of all we have left. Hope that things can and will get better. Hope that the struggle doesn't last forever. A sigh a relief after distress. The fight against the odds that causes no harm. An exhibition of faith and love.
We have to learn how to have the right outlook on life.
I'm not saying to live in oblivion or ignorance. I'm just saying to not be afraid to want more and to expect the best. You should always feel as though you deserve the best.