I see as one of the main elements to this, is God at work, You will not be tested more than you can handle. To me it is the Hope and assurance of a life in eternity as an heir with Christ. This life is only temporal, a very short time and the older I get the more I have come to realise this.
Romans 9 is a very import Chapter, God has chosen you to be with Him for Eternity, and for a purpose for His Kingdom here and now. regarding mercy, it is very true that God has mercy on who he wills. this is very important, for no one deserves God's mercy, Paul argues this point in Romans 1:18 - 3:20.
I find it truly humbling that God actually has mercy on any.
Importantly, it is through this mercy that we are able to exercise Faith, for God softens the hearts of of thpse whom he chooses to have mercy on. remember in Jesus' own words we are condemned already. God chooses to have mercy on those he chose to glorify His own name.
Men think wrongly when they believe that they in their hardened hearts, living in darkness, slaves to sin, decide all of a sudden to choose God. if it where up to man, no one would choose God.
Throughout redemptive history, in other words all through scripture we see that it is God bringing people to Him, it is God being Faithful, it is God calling, not man.
So to get back to the question, it is the determinate will of God that His will be done, to the glory of His Holy name, through those he had predestined to further God's redemptive plan, and to glorify His name to all peoples of all times and ages.
In Love
Phil