trust me... i do get it. i've had the same experiences as you many many times and still struggle with this exact issue. people are flakey and even the best of my friends seem to be constantly "unable" to honor their commitments for one reason or another - even though they seem to be able to do things with either people.
I do not have all of the answers, but i think it's important to consider the context in which you interact with people. it's kind of a new philosophy i am exploring... I know that it is way easier to be friends with people whom you see on a consistent basis and have current things in common with you, such as classmates, coworkers, or family, than it is to be friends with someone who you would not see or talk to without specifically trying to.
Other people seem to be able to make many friends with a wide range of people that they work or study with, but I tend to be much "pickier" with friends because i don't get along well with EVERYONE. i look for specific traits and personalities that i know i can handle and can handle me. So i find that I'm often trying to form or maintain relationships with people who i no longer have a specific activity in common with. Sometimes i get offended when a friend can't honor a commitment to me, but if i stop to think, the people that they DO see and talk to are actually those whom they have class or something with and see all of the time. a lot of times, they just go out after class or after work. sometimes they get together to work on a project and then they end up watching a movie or something. it's a reasonable way for people to function with those who end up around them all of the time, it's not like they are making special plans with all sorts of people except for me.
i know this doesn't answer all problems, and there are still "friends" who blatantly are rude or conveniently too tired, too busy, etc, but hold a complete double standard during interactions with others. i know it hurts like crazy. but this is just food for thought...i think it's important to try to understand and give the benefit of the doubt, just as we hope others may do for us (even though they don't always).