If you are going out and travelling (and not just on vacation) any meaningfull trip will cause some second thoughts. Most people that are making you feel guilty or are making you question yourself, simply do not understand where or why you are going! Also, there is a jealousy factor as well. Many people want to support you but personally can't, because they themselves could never do what you are doing. The bottom line, is that any decision you make in life that has any meaning will make you think twice. Those for me, have turned out to be the best choices I have ever made. I would not give up any travel experience I have ever had.
boreal2673, with all due respect you're attempting to project a lot of your own personal emotions on a pile of people you don't even know.
Claiming that any meaningful trip will always cause second thoughts is nuts. There's loads of travellers who don't question their "meaningful" trips for one nanosecond.
Different strokes, buddy.
I'm thinking it's just the excitement making you restless and think about lots of things. I can't say it's your parent's fault either - I mean, they're obviously just concerned. You already said it - they're worried because of the same reasons as to why you're having second thoughts. If the "spending guilt" really troubles you, try to cut down costs when you get to the destination. You should just try to get the best out of your travel and make it memorable and worthwhile.
There is so much to see in this world..Your parents should be a little bit more concerned if you bought a one way ticket to Uganda or somewhere in africa, San Fran for one week, should be easy.. Just don't think about it and do it, there are so many people that literally do nothing with there lives and we can look upon them and realize how awesome our lives are! and of course everyone has second thoughts but the more trips you go on the easier it becomes.