It is possible to know. If God lets you know. It IS a grace that He can choose to give, and He has in the past.
Always strive for a closer union with God. He let St. Faustina know, for example, that she would not be judged and to no longer fear the judgment of God.
Another way to help secure your Salvation is to always be merciful and do acts of mercy, especially spiritual acts of mercy. Jesus told her that if people knew how to store up treasure for themselves in Heaven, they would not be judged, because their mercy would forestall His judgment.
Always be looking to do the Will of God.
If you are looking to do the will of God always, everywhere, and in everything - you are doing the same thing that everyone else who was predestined was doing!
Be certain of this:
God opposes the proud and gives grace to the humble
Always fight pride as if you were fighting the Devil, because sometimes you might be.
And remember the words of Our Lady:
Luke 1:50
AND HIS MERCY IS UPON GENERATION AFTER GENERATION TOWARD THOSE WHO FEAR HIM
Trust in His Mercy.
As for Eternal Peace with God - yes it is a necessary door. We sow now. We reap in Eternity.
Even if He lets you know, you still have to go through that door to get to Eternal Peace with Him.
Hope and Trust. Always seek to do His Holy Will, which is Love and Mercy itself. Hope and Trust in Him.
Remember that God is Love and He literally loves you. Literally, and infinitely. God is infinite and God is Love.
I struggle with this too. Ask God to give you comfort and to help you trust. He always helps me. Sometimes I have to wait just a little bit, but He always helps me. I just don’t trust Him like I should. Pray for me please.