You must lighten and remove any unwanted undertones before you can colour your hair lighter. This step is not optional. If you do not take the time to remove what shouldn’t be there, you will see it lurking under the surface ruining the effect you were going for. Your classy platinum blond is going to look like a discount bin brassy box colour. It can take time, and it might be expensive, but it can be fixed! Of course, you can always start fresh from scratch by cutting your hair off to 1” of fresh regrowth, but I think that is called Rock Bottom and you probably want a softer touch to improving your life and hair! The second step is lightening.
If you can not remove something (now) say, a horrible boss, how can you lessen the stain that work is? Of course, our goal is to bleach out bad influences completely! But, realistically this will take time, and you may not be able to do it all at once, but do not use this as an excuse to not remove what needs removing. Remember, if you only lighten and skip toner the undertone can still stain Your Perfect Life.
What do you need to get rid of in your life? Bad habits? A frenemy? Clutter in your house? Take some time to reflect (and if it suits you journal) your thoughts. What is preventing you from being your best self? What is dulling your shine?