I get the perspective, though it seems to me that the only way to rectify a belief that salvation is eternal upon conversion(which would then take defining when that conversion moment happens, which it seems to me people fall away with all sorts of genuine looking stories) is if we somehow lose our free will.
Jer 31:31 Behold, the days are coming, declares the Lord, when I will make a new covenant with the house of Israel and the house of Judah,
32 not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt, my covenant that they broke, though I was their husband, declares the Lord.
33 For this is the covenant that I will make with the house of Israel after those days, declares the Lord: I will put my law within them, and I will write it on their hearts. And I will be their God, and they shall be my people.
Two important points that are being made here:
1) God wanted a covenant to function permanently and not one that fails because people break it by lawlessness. If the new covenant can be broken by lawlessness, it is not any better than the old. God states this will be different. (God can't host lawlessness though, hence point 2)
2) Their hearts will be changed in such a way that they walk the walk. Which is why the covenant will not fail/be broken because God surely isn't the one to fail, it's us.
The real question, like you said, is who is truly saved. Only God knows this and the person may inquire God for themselves. We were only given some clues to be able to know to know things to a point in order to protect the church from the wolves, like to look at the fruits of the Spirit.
unless you're preaching that God does absolutely everything including willing for us then there isn't a reason for complete confidence
Predestination and free will are harmonized through foreknowledge ability of God (1 Peter 1:2). God already knows what beings He designed will choose, given all their influences. One cannot break the other, or the Scriptures, since they preach both, would be broken. So I believe we have free will but it cannot break God's plans, and God does what He pleases with us being the potter and shaping us as clay, but it doesn't negate free will.
(I have to accept that I will never fully comprehend this with my human mind.)
Be careful to the one who thinks he stands, lest he should fall.
It's warning against arrogance. Falling means falling into temptation here, because the very next verses are about temptations.
God will always reprove arrogance by hammering it down some way, it's what God does.
Is it really arrogance to believe God holds me securely and that He will always lead me until the end of my journey?
Does God want me to stress all my life? I had quite a lot of anguish about losing salvation first few years of being a believer. Sometimes I couldn't sleep for days and when I did sleep, I'd cry myself to sleep in great despair and have nightmares. I don't see how is that the peace of Christ.
And if God does do absolutely everything, then it becomes requisite that He is also the one willing that some be damned which goes against the heart of grace.
2 Timothy 2:20 But in a great house there are not only vessels of gold and of silver, but also of wood and of earth; and some to honour, and some to dishonour.
"Vessels of wrath prepared for destruction", too.
Romans 9 addresses in depth what you wrote above (Specifically that pre-ordination does not mean God wants some damned)
And it comes down to conceiving of salvation purely as a past reality, which i don't think is the only way the Bible teaches it. It is both now-but not yet
I think you're confusing salvation with sanctification process that unfolds after one is saved.
We're less likely to respond to the Shepherd's hook bringing us back to the right path if we think the path we are on is a sure salvation.
This is human logic and estimate how things work.
Spiritual things don't operate logically but rather confound logic.
The law, which is by logic supposed to bring order, enabled sin. (Rom 7:5, 1 Cor 15:56)
Forbid something, and people are soon dying to do just that.
In the same way, grace, not keeping a dose of "healthy" fear, is what leads towards perfection:
1 John 4:18 There is no fear in love; but perfect love casteth out fear: because fear hath torment. He that feareth is not made perfect in love.