Can I use these interchangeably?
Now I will live a life I have dreamed about for years.
Now I will live the life I have dreamed about for years.

Question

Answer

Hi Sinem,

A life indicates one of many. 'A man' is one unspecific man from many. 
The life indicates one specific life. 'The man' is the man specifically spoken about.

So 'Now I will live the life I have dreamed about for years.'
Although both can be used, this one is a bit more specific and fits better.

Hope this helps.

Aleks