What to do if your mates let you down all the time ?
Let me guess... they ditch you cuz their girlfriends want to do something? It happens all the time... Go out and find yourself a gf, then you can all do stuff together.
find new mates, if you are always there for them and they are continually letting you down, they are not your mates, they are using you, when it's convenient for them.
find another mate!