The bible teaches that Jesus'promises are true, not that predestination is true.

Because God is not a liar when Jesus says something that's the ultimate authority for it happening. He promised curses to the rebellious and blessings to the meek. Is Jesus MAKING someone rebel against him so that would place themselves under his curses? That sounds like Jesus would be double-minded.

But if Jesus says that these certain things will happen, and these certain people are guaranteed salvation, that just proves his promises are true, not that all things are predestined. If all things were predestined then there would be no free will and no choice but we see and know that we have free will and choices.

A guy I know said of some people he knows, It's not that I'm better than you it's that I make better choices than you.

Make better choices.

To visualize time and why not all things are predestined read this thread I made which explains how true time works:
Thread: I solved the time machine paradox with bible-based theories
In summary, the past ceases to exist as soon as it happens, the future is fluid to our choices. If you went into the future and came back you could change the future. That's the equivalent of knowing the fate of sinners and continuing to sin believing that you're predestined to continue in sin, whereas it was a choice all along.