Hi there!
You're background is very simillar to mine! I'm 20 gave uni a try and always wanted to be cabin crew! I took the plunge and now work as a full time cabin crew member. I think the best thing for you to do is to start travelling. Once you start working for an airline you will hardly have time to scratch your own arse let alone travel imo! Also the novelty of travelling will soon wear off and i'd say go and enjoy it while you can. I wish i'd have travelled a bit before i started, but i was quite well travelled as a youngter so i don't regret it too much!
I very much doubt the airline would hold it against you if you don't go to the interview. At my interview about five people didn't turn up and apparently it happens all the time so no worries there. Which airline is it for out of interest?
I don't know if that's been any help to you, but i tried! Good luck in deciding!!