Jeesh! You say there is no point to life and that your are depressed. But then you say that if you ould get famous you'd be happy. You REALLY need to sort your priorities out. Yes I know thats not what you wanted to hear but there you go.
On another note, university is actually a very good time to start trying to break into new industries. There's bound to be a drama club etc. at your uni and besides uni is hardly a 24/7 thing. You'll have your own money, you'll have your own space and time-what better time to start exploring?
The main point to realise is the entertainmet industries require talent. You cannot be taught to be a good actor-teachers can only sharpen a talent you have in the first place. You've only got to look at how many drama students are actually succesful. I would go as far as to say that in many ways, drama lessons are useless; the best thing you can do is simply put yourself forward for university plays etc. and get used to the pressure of it all.
All the drama students I have met have actually been useless. They have this idea that by committing to a drama course/school, somehow thats enough because they are 'serious and determined.' But they seriously are rubbish! Infact, I think pretentious is how you'd descrbie 98% of drama students. The best actor I've known was actually geography student. Put simply, no one respects a drama student-but people do have respect for those pursuing drama whilst studying another course. So actually, not having experience can be a benefit!
But yeah don't give up-your life is far from resigned to a miserable job in a office at 18. Your still very youg, have little responsibility yet, full of potential and have plenty of oppurtunities facing you.