Should I go through this again?

Some in the Yahoo Answers community know about my situation, some don't, so I'm just gunna sum things up a bit. There's this girl I've known for over 3 years now, right now I'm 17 and she's 15, we've tried a relationship with each other numerous times. It seems like every time things get really great for us and goes our way, she doesn't like it. This last time we tried our relationship, we weren't even together a week before she was CHEATING ON ME!!! well, I stayed with her for nearly a month AFTER finding out about the situation AND confronting her about it. I stayed with her because she SAID she was going to choose who she wanted to be with soon and I was stupid enough to believe she would pick me apposed to this *** hole she hadn't known for longer than a month that didn't even know how to make her smile. I've always been there for this girl, even when she was with someone else before and it hurt me so badly, but i was always there for her. she always calls me crying her eyes out because of something that happened to her and i'd always be there to make her happy, ALWAYS. I went with her and her family to the beach recently and her dad just fell in love with me because of how great i treated his daughter, that's how well i treat this girl, seriously. ehhh i forgot a huge important detail, she lives 300 miles away from me, heh... But i'm suppose to go to her house july1st and stay with her for a week, she wants me to smoke pot with her (i'm the stoner, i've been smokin for 7 years, yes that means i started smokin marijuana at the age of 10, dont worry, mom didn't know, it wasn't bad parenting, it was just opportunity) she's smoked pot before but "only once" is what she says, but she wants me to smoke pot with her and have whenever she wants it (15 year old craving , yes, the world is coming to an end it seems like) and yes she is my ex girlfriend. well within the past month she's broken up with that guy she was cheating on me with, and is now dating this guy that she's known for like ever and whom of which she's promised to me before that they could only be friends. She STILL wants me to come for that week and she STILL wants to smoke pot with me and she STILL wants to have with me, even though she doesn't want me smoking pot at all for ANY reason except if she's smokin too, she has a boyfriend but still wants to have , should i go to her house? i've promised her already that i would, and we're stayin with her dad because her mom hates my guts, and me and her dad are good friends and we hardly ever get the chance to talk or hang out. What should i do? i dont want me and her to be "friends with benefits" i would rather us either be friends or in a loving relationship that'll actually last, i'd prefer being in a relationship with her, but i only want it to be me and her, no one else, i want us to be happy with each other like we use to be, but she's changed so much, she's out there to only make her self happy, or what she thinks is happy, all she's doing is getting temporary joy, there's no lasting happiness with any of her choices lately... what do i do?