the experiences of all the other frustrated people will not necessarily be your experiences. don't be put off because the guy beside you has failed.
a parallel to your question is the concept of 'how do you own an aeroplane?'
aeroplanes are very expensive. no one has enough money to buy one so how do you get to own an aeroplane?
the answer to this was written by an american about 50 years ago and it is still valid today.
Want to!
you see if you want to then you will make decisions along the way that will gradually make it happen.
you say that air cadet gliding is hard to get into. have you applied? if you have never even tried then you will have no chances of succeeding.
getting into aviation without spending money is a futile dream. flying has costs but getting into aviation without wasting money is certainly achievable.
the key is to want to. if you really want to and put in the effort you will slowly make it happen.