Life rule number 2: Life is full of disappointments. Get used to it.
At first glance, this may seem like an extremely cynical and pessimistic thing to say. But you haven't yet allowed me to fully explain my position on this. I think that the sooner we realize that there will be disappointments, the sooner we can get over them. There is no point in dwelling on false expectations, broken promises, or hopes lost in the wind. It's much easier on you, mentally, emotionally, maybe even physically, if you are able to keep some thick skin and not lose it when something doesn't go according to plan, or something doesn't go how you thought it would.
Disappointment will be a part of everyone's life, and no matter how prevalent it is, it's going to suck, plan and simple. That being said, learn to deal with it early on. It's not going to help you one bit if you can't keep your head on straight at the first thing that falls out of line. Lives are not built on perfectly sculpted pathways lined with flowers and adorned with statues. There are paved roads, dirt paths, and sometimes you will need to forage through the forest because a felled tree is blocking your way, or the swamps have grown unexpectedly large this year.
Sometimes people don't know what to expect in life. But if you're ready to embrace disappointment and move on, you'll be a lot better off in the long run.