Minecraft won't let me log in? - 1

I've tried logging in many times and i'm sure i have the right username and password but when i go to log in it says i have the wrong password i have changed my password many times the game and it has only just let me log in although now its saying i can only play the demo because i haven't bought the game when i bought it a really long time ago i've tried on two different computers and it still doesn't work.
WHAT SHOULD I DO:'