hi hank what i dont know if a lot of guys realise is the amount of effort most gf's put into pre-date preparation - they wont tell you this but all the deciding what to wear/shopping, getting hair done, nails etc etc can start like 2 days before the date easy, it takes me an hour just to blow dry my hair!, and, all this effort is essentially for you so that they look nice when they are out

so when you cancel on us, it really can make us mad just cos its inconsiderate basically and you are saying essentially you dont respect her and she (or her feelings) are not important to you. perhaps you didnt even think maybe she cancelled her 'cool stuff' so that she could go on a date with you??
when she acts like she doesnt care, its not that she doesnt care, its that she doesnt want you to see that she is hurt, and she is pretending that she isnt even bothered that you wanted to do your 'cool stuff' rather than see her.
so think
why are you annoyed with her?? isnt it good that she has some self respect? if you need to do some other stuff and need to cancel on her, then maybe dont leave it till the last minute would be better, or plan your time a bit better so you can still do your cool stuff and see her. what you are really saying to her when you do this is being with your gf isnt as good as the 'cool stuff' you are doing. its like you are saying 'if nothing else more fun comes along this weekend, i've always got my gf as a back up plan'
so...advice (not from a pro) how to deal with this; i dont think you should be 'cool' about it ..imho if you want to keep her you should simply apologise to her for messing her around and tell her you respect her. of course you should both do stuff with your mates and spend time apart from each other, but if you keep cancelling on her you either
a) wont have a gf anymore cos she will dump you for other more cool guys who appreciate her and make her feel like they put her first, or
b) she will stick around but end up resenting you but not leave you because she has no self esteem, and neither of you will end up happy in the end
i guess it depends how much you care about her basically and if you do, whether you want her to know it or not