My friends are being mean! :(?

Kay so since before i always went home by car but now school is ending and its really warm outside so i just walk with my 3 friends, don't wanna use real names but here lets call them, Allison, Mandy, and Jenna, and so they go to this gas station every wednsday and buy slushies, they said its their tradition, and they said i cant go with them! Even though its ALWAYS since the begining of the year been the 4 of us, they say its their thing and i cant join them! Now Mandy said that tomorrow (wednsday) they can go get slushies and then go to mandys house and go swimming, even though jenna doesn't know how to swim. Mandy has always been the nicest one! Shes suppose to come over on thursday and watch some movies with me, anyways she invitied them but she didn't invite me! Should i invite myself? I really want to go and please don't tell me to find new friends! What should i do! :( Im so sad.