Parents are anti trip

So pretty much my parents are very anti my going on this trip (more so my dad than anyone else). Ive been saving up to go on this trip ever since i decided to go. Im just over half way there and with a birthday coming up I am hoping to be close to my target by at least september (planning on going on the November Grand Southern trip). I've been talking heaps about it with my parents and both think its a waste of money. I have been to America twice once when i was little the other 2 years ago, but that was with family and we only went to California, Las Vegas and New York, and I really want to see alot more of it.
I think its something that would be an amazing experience, but my dad hates the idea and seems set on his ways. Apparently wasting my mony on going to a country i have already been to is silly, and that I should better spend my money on something like buying an apartment, or using it for my car. I understand that but this is something ive been wanting to do for soooo long, and ill be 19 on the trip so it seems like the perfect time to go.

I dont know. I want other peoples opinions. How can I change my dads idea of it being a waste of money.
Also i was wondering on the best way to save my money? So far ive bene pretty good, but whenever i enter a shopping centre the shopaholic comes out in me and i go crazy and spent hundreds of dollars. I've now avoided the shops unless my mum is there to stop me impulse buying. I've also cut down on my social outtings, and when i do go out i dont drink (because last time i did it cost me a fortune).

All advice and comments would be great Thankyou!!!