“You will know it [enlightenment] at the latest when you feel death approaching. Death is the stripping away of all that is not you. The secret of life is to die before you die” – Source: Eckhart Tolle
What Tolle is getting at here is how to understand the difference between the ‘I’ and the ‘self’. The ‘self’ lives in the world of illusion and the ‘I’ observes. The ‘self’ worries about things that are illusion (like what do other people think of me?, Will I get a job that will make me happy?, etc.).
If you can separate the two and realise that most of what the ‘self’ craves for are goals set by society and so in our imagination (illusion, not reality) and learn to let your ‘I’ observe your ‘self’ so that it can see depression (etc.) but know it is passing then you will be happy.
People who are at the moment of death suddenly realise all the things they thought they needed were not that important after all. If you can reach this point earlier in life you can enjoy far more.
See Understanding illusion and reality for more on this.