Ibn Qayyim al-Jawziyyah (rahimahullaah) says:
“The one who entrusts his affair to Allah does not do so except because of his wish that He decrees for him whatever is good for him in his worldly life and his hereafter, even if that which is decreed for him is contrary to what he thinks would be good. So he is pleased with it, because he knows that it is good for him, even if the good aspect of it is hidden from him.
And (the slave) is certain that his ability is in the Hand of Allah, not in his hand. So He is its Owner, not him, for if He did not give him ability, he would be incapable. Hence, he does not move except by (the help of) Allah, not by himself; so his salvation is only by Allah, not by his work.
So the one for whom Allah decreed something, and allotted a share of provision or obedience or condition or knowledge or other than that, then it will inevitably happen to him; and the one for whom that has not been allotted, then he absolutely has no way to it, just as he has no way to fly up to the sky and to carry the mountains. So in this same manner, he cannot oppose what has been allotted. Thus, whatever is for him from it, it will come to him despite his weakness, and whatever is not for him from it, he will not attain it (even) with his power.”
Source: Madaarij us-Saalikeen; At-tafweedh, Ath-thiqqa billaah; Volume 1, p. 542-546