She has a lifetime want to get to the top of the education career which she hasn't done and she has the memory that she's achieved her lifetime want and is platinum. How do I fix this? It's happened twice and I haven't saved the game either time it's happened.

That's the Free Time bar. You might want fragile permaplat, I've found that quite good at tossing my born in teen permaplat sims back to normal. http://cyjon.net/node/345 and or slower aspiration gain http://www.cyjon.net/node/404 since that bar is tied to aspiration gains. You can use SimPE to remove the platinum or some mods have the option.

Pescado's Lot Debugger, AKA the Batbox, has a Toggle option to set or remove permaplat. It's not straightforwardly named that; I believe it's Toggle...LTW Aspiration or something like that. It can be found at More Awesome Than You.

It is not a fault, it simply happens because your Sim has achieved enough points in other ways - so by fulfilling the normal wants of Sims, you also add to aspiration points.

So basically if they gain enough aspiration points they can become permaplat regardless of whether they have achieved their lifetime want or not?

Exactly. If you look at the wants in the panel, some have blue frames - they add aspiration points while the others only add influence.
Some Sims will then achieved platinum regardless of LTW . So it just a normal part of the game.
There are mods if you don't like it - I personally do, so I cannot direct you, but you can use the batbox.