Help me become platinum!

The top part of your hair (above your eyes) looks platinum to me already, but you have some orange/gold in your lengths. What is your hair history, were those parts previously dyed? If so it could be old dye in which case a colour remover will help. It could also be staining, which is permanent, so your only option will be to stick to darker shades of blonde on those parts.
If it was virgin hair when you bleached it, you will need to go through it section by section and spot bleach the dark parts. Either a low vol bleach or a bleach bath, but you will need to avoid getting the bleach on the areas that are already light enough.

Last year I've bleached my hair a few tones ligher than my natural hair and it was too orange (first time bleaching, didn;t know what I was doing at all :D) and I dyed it dark brown (but it was still lighter than my natural colour). I guess I really should try colour remover then. But I thought bleach would do the same? :< I am not sure if I could find  colour remover (living in a small town sucks) and I can't order anything online right now 🙁

And what would happen if I accidently bleached lighter parts? Because I thought I would do my regrowth first, wait a bit, when all that darker hair and then I would put bleach all over my hair. I also was thinking about putting coconut oil before and I guess it would be even harder for me to spot those darker spots.

It is actually really difficult to bleach permanent colour out, it tends to stick at that orangey stage. So try a colour remover if you can find one.
Bleaching the parts that are already light enough is really risky, it will cause unnecessary damage, possibly breakage.
If you want to do all the hair at the same time (roots and dark areas) you can mix different strengths of bleach and apply as you go through each section. There is no quick fix for this, the only way to get it even is to go through it and lighten each piece to the same colour.