It's a good thing when those we depend on the most, suck the worst:

Why does it seem like those we count on the most, miserably disappoint us?
The people we came to depend on the most, ended up sucking worse than we ever imagined possible.

Consider this when rethinking your last major disappointment:
It's unrealistic. No, we are unrealistic.
We put too much confidence in others... No, we've discounted our own ability.
People are selfish... No, we are giving into selfcenteredness.
Other people aren't giving me what they owe me... No, we've tried to shuffle off something we should take responsibly for.

I could go on, but I think that I would bore you if I took this much further.
There is a perspective-issue on the line, and it's uncomfortable to come to grips with, I agree. 
People let us down, important people in our lives who weren't supposed to let us down... and let me say, it's okay.

It's okay, because of two reasons:
1., We need to rely more on ourselves & on God.
Not to say we are an island or meant to be isolated, I believe in being interdependent.
I'm saying, we often relegate too much of our self-esteem, self-confidence, and, we don't take enough responsibly for ourselves.  And to the God thing, we idolize people when we give them too much sway or power over us.  When our expectations aren't met, we act like the other person had some sort of divine powers, and they cheated us out of a miracle. 

2., This is good, because when other people let us down and disappoint us, it's a reminder of how often we fail others.  We do.  Admit it.  We make commitments and promises, and we don't follow through every time.

Sometimes we have to quit looking to everyone else, to bring us a sense of happiness through their performance.  Sometimes, we need to realize that there are people in our lives that simply can't get healthy.  They are doing the best they can, I guess.  Who cares?  If they can't "deliver," then we need to move on with our lives.  If they ended up being selfish, which it looks like, move on and learn how not to be.  We can learn a lot about how we ought behave, by observing the way we feel when people mistreat us or leave us hanging...

We will have mentors, relatives, co-workers, close friends, all who we will depend on in one way or another,  but let's not overly depend on them.  Avoid the unnecessary pain.  Try to see the best in others, hope for the best, but remember -- all that glitters isn't gold...