'Thuder down' can be simply understood as 'crash down'.
Some people acheive great things with less effort or just by sheer luck while most of us have to work very hard to achieve anything in life. Sometimes it may seem unfair when you've put all your effort on something and it thunders down without any success.
There is no denying that life can be unfair and cruel. But don't let it discourage your confidence and thunder down your spirit. The real winner is one who does not give up to the challenges life throws at him/her. If you aren't born with any luck, go make some.
