I've finished highschool recently, and since about 2013

>>16774547
As far as the computer goes, this might just be a phase you're going through, or you might just be feeling a bit down and loosing intrest in things for a while. I think you should still build it. It would be a shame to miss out on something you've been looking forward to for so long, and you never know, it might cheer you up a bit. Plus, even if you don't actually play games on it, a powerful pc is never a bad thing to have. No one ever sat at their desk and said "man, I wish this thing took longer to boot up" or "that's a shame, I was really hoping it would crash and loose all my data when I try to open Word and Excel at the same time".

Perhaps go for an interim build, something that will do for now but leaves you with the option to upgrade later if you want to (I just did something similar for my oldest son). Buy the case, psu, and mobo as if you were going to build a top of the line machine. Throw in an i5 4xxx, one 4gb stick of ram, a gtx 750ti, a single 1tb hdd and just use the stock coolers on everything. That'll keep the cost down quite low and give you a reasonable machine for now, and if you decide later on that you want to run an i7 6xxx, a pair of titan z's, 64 gb of ram and three ssd's in raid0 you don't have to start the whole thing over from scratch.

>When I stop playing games for good, what interests could I take up that aren't extremely expensive and are actually useful?
Not all interests have to be useful. Or to put it another way, having fun is useful, because it makes you feel better. Personally I like fishing in my spare time, but it's not for everyone.