How Do You Learn To Say Fuck 'Em

So I have been feeling left out by my friends alot lately. I have even tried talking to some of them about it. They have all said the same thing, they aren't leaving me out, it's just that things come up at the last minute and they get caught up and forget to call. Well I would like to scream BULLSHIT!!! I happen to know for a fact that my friends have been at parties (thrown by other friends) the last two nights and that there is another party planned for tonight. Granted, tonights party was a last minute thing put together last night, but yet has my phone rang today? No. I only know about ti because another friend (not the one throwing the party) asked me to come. Now I can't just show up there without the person who's party it is calling me and inviting me. So now I know that they either don't want me around, or they just plain don't think of me. I don't know which is worse. I do know that someway somehow I have to learn to say fuck 'em!