Believing in Allahs qadar

Asalamu alikum 

Do ppl really believe in their whole heart THE QADAR OF ALLAH ..do they understand it's really meaning

Qadar means that Allaah has decreed everything that happens in the universe according to His prior knowledge and the dictates of His wisdom.

In Saheeh Muslim (2653) it is narrated that ‘Abd-Allaah ibn ‘Amr ibn al-‘Aas (may Allaah be pleased with him) said: I heard the Messenger of Allaah (peace and blessings of Allaah be upon him) say: “Allaah wrote down the decrees of creation fifty thousand years before He created the heavens and the earth.”  
And the Prophet (peace and blessings of Allaah be upon him) said: “The first thing that Allaah created was the Pen, and He said to it, ‘Write!’ It said, ‘O Lord, what should I write?’ He said: ‘Write down the decrees of all things until the Hour begins.” Narrated by Abu Dawood, 4700; classed as saheeh by al-Albaani in Saheeh Abi Dawood.

So why is it then that when something goes wrong ppl start to think that what ever has happened happened because they didnt do such and such or because i'm no good ... the truth is the qadar of Allah is just that ...he has written it before you were born ... the same goes for Polygyny ... all of what will happen in your life has already been written .... and when a man takes on another women then that was written for him as well as for her she may have been married before but whatever happened in her marriage happen because Allah decreed it to happen at that time because maybe she was meant for another

I think if you dont believe full in the Qadar of Allah then you have not accepted the full extent of islam ...because the Qadar of Allah is one of the Pillars of Eman

Allaah says concerning His actions (interpretation of the meaning):
“And your Lord creates whatsoever He wills and chooses”
[al-Qasas 28:68]

“and Allaah does what He wills”
[Ibraaheem 14:27]

“He it is Who shapes you in the wombs as He wills”
[Aal ‘Imraan 3:6]

And He says concerning the actions of created beings (interpretation of the meaning):

“Had Allaah willed, indeed He would have given them power over you, and they would have fought you”
[al-Nisa’ 4:90]

“If your Lord had so willed, they would not have done it”
[al-An’aam 6:112]

All events, actions and happenings occur only by the will of Allaah. Whatever Allaah wills happens, and whatever He does not will does not happen.