like just from an storytelling perspective, what are you actually doing both a) when you are rolling your fortelling rolls and b) when you use one of your fortelling rolls? is the vision happening just prior to you using the ability? Are you seeing visions of the times you are about to use your fortelling roll during your sleep, forgetting them when you wake up and remembering them when you use foretelling roll? The ability just kinda seems a bit like an arbitrary meaningless dice mechanic, and i would personally apreciate it if we could use these portent dice to gain reliable information about the game world, with the exact information gained being based on the roll so for example, maybe if you choose to spend a roll of 10 or lower you get to experience a past event, with lower values being events further back in time, and spending a roll of 11 lets you see a future event, with higher values letting you see further into the future, even numbers shows positive events and odd numbers show negative events or something similar to that.

Your problem is you are looking at the game mechanics and trying to work backwards to the story.  That's backwards.   Instead look at the story of a wizard seeing the future and changing it to his liking. 

For example:   You know exactly where the bad guy is, so ahead of time you tell the fighter "Go for his belly button,  NOT the Head."  Or variously, "The rogue is going to duck under the fireball, so wait half a second till he pops up THEN fireball him."

But merely having this advice before hand is not a certain win.   So instead of the chronu wizard's auto-success, you get extra rolls.   You have two visions, so you get two rolls.

But as we can not REALLY see the future, we have this awkward roll twice before hand and choose when to use them game mechanic.

My problem, RP wise, is that the feature requires significant metagaming. You roll d20s on completion of a long rest which are to represent your portent dreams. However, later in the day you can use the d20 to override a roll -- instead of the enemy rolling an attack they use your d20 portent result, for example. This is "the event matching what you dreamt".

This is basically you as a player saying this event is what your character dreamed about hours before. This makes it a great mechanical feature but a lousy roleplay one because if you woke up with prophetic dreams you're not going to keep quiet about it, but in this case you have to because you can't say what it is now, you have to "retcon" later.

Something like premonition should be a "call on the DM" thing. It should be something similar to spells like augury or commune. You should, there and then, get some info about something possible that the DM may have planned. OR the feature should more like forcing advantage or disadvantage "at the time" because you had a quick premonition just moments before, similar to what Foresight does, not something you dreamed hours ago and kept quiet about for whatever reason.

Personally when I play a divination wizard I ask my DM to reflavour the RP of this. It isn't something I dreamed in advance but rather my wizard affecting probability in that moment and pushing a more favourable feature. It's still a bit clunky but makes it sit much easier RP-wise.

I kind of thought of it as the scene in the matrix where Neo sees the black cat twice “whoa...deja vu”. It was him using his portent dice on a perception check and realizing the matrix was changed.

its not like you have a dream and you know it’s going to happen and you keep it to yourself all day but more that you had the vision but it is elusive. But just as you are about to step on the floor trap that drops 50’ to spikes below it comes to you “like deja vu” and you use the other die roll and safely avoid the trap. 

that’s just how I interpret it for RP. 

Edit: you could even RP it like just as you are making an attack roll (say, casting an attack spell that ends up missing) your eyes roll back into your head leaving only white visible and almost as if in a trance you weave the spell and the spell unerringly hits its target