Once Saved Always Saved is a doctrine that means different things to different people.
Some take it to refer to God’s election–if you are elect before the foundation of the world, you will remain elect forever.
Others tie it in with conversion–if you say the prayer, or get baptized, you are always saved.
Some have, in a supposed effort to elevate God’s grace, gone so far as to say that a believer can fall away, completely turn away from God, even become an atheist, and yet, if that person said the prayer when they were four, they are still saved.
Most, however, upon seeing someone reject their faith, completely turn from God, will conclude, “well, they were never saved to begin with, even if they thought they were.”
Once Saved Always Saved is there to grant assurance. But if someone thinks they are saved, then turns away proving they never had faith, how is there assurance for anyone else?
How do I know if ten years from now I turn from my faith, am I just playing a game today? If I die today while not having denied my faith, but would have if I were here another ten years, am I saved or not?
Mind. Blown.
There are a handful of verses that seem to fully support Once Saved Always Saved. Then there are a handful of other verses that seem to completely eliminate the idea. There are even a handful of verses that seem to back up the “well, they weren’t saved to begin with” idea.
I think our doctrine of security should deal with all the verses, not just the ones that lean to the side we like. This is no easy task.
anti-itch meditation 
This can be quite a confusing subject with as you say verses that tend to support differing viewpoints. To add to the confusion I’ve seen people backslide over the years and some of these people repent and return to God, whilst sadly others don’t return. I’ve always wondered why some return and others don’t. Puzzling!
Yeah, it’s tough. Doctrine is not supposed to be based on experience, but doctrine should help us understand experience. Perhaps all the different verses shed light on all these individual experiences, and none are to be taken categorically?! I don’t know, that freaks me out as a conclusion!