UGH I'M SO MAD! PLEASE HELP! NEED TO RESPOND!!!?

My friend is having a birthday party at a water park and I'm on my period right now. I really want to go to that party. It will be sooo much fun and I'm so glad that she invited me b/c I thought she didn't even like me that much but she does and I REALLY wanna go b/c I never get invited to parties and I don't have that many true friends. Anyway, I've had my period since the 22nd. When I woke up and went into the bathroom I saw that I got it. The party is this Thursday. BUT here's the big thing I'm worried about. Well first, I thought that I could see by Wednesday night if it is over yet, but now she wants me to come over her house to swim and then sleep over and then her mom will drive us and the other girls to the water park. I can't quite get the hang of putting tampons in yet because whenever I try to put them in, the plastic doesn't go up all the way. I don't know why. Anyway, the last time I had my period it lasted for about 8 days I think. So I don't know if it'll be over by then. But if I don't figure out how to put in a tampon by the time of the party, can I just put a liner on with underwear, my bathing suit bottom, and shorts? Or will it leak or show through my pants? I'm so mad! Please help! What do I do? I hate myself. I can't do anything. I AM GOING TO THAT PARTY! If I still have my period... I don't know. I need to figure something out. Help PLEASE!!!