what's wrong with me! please read and help me!

So I have my best friend called issy and my best guy friend called kurt! And well I'm really close with them both is what I thought! But then kurt invited issy to the semi and not me! And I have literally been their for him and I obviously thought he would take me! Like I didn't care who he took but the fact it's issy, literally made me depressed! Then issy was like I feel bad, like he should of taken you! And I was like it's fine, you really wanna go( it was not fine, please decline him!) she was like omg I need to book and find a dress arghhh! And I just feel like absolute shit that my best friend didn't realise that I'm so hurt right now and kurt took her! And then when I talked to him about it, he was like just thought she would be a better option!!