Hey, I really need some advice!

Hey all, I've joined this forum in the hope of getting some advice on colouring my hair.

I've always wanted platinum blonde hair, my hairdresser wouldn't do it for me because she said it would be too damaging. My natural colour is a dark mousey blonde which she lifted to a nice lighter blonde. I wasn't happy at the time though because within a couple of weeks the colour would fade to an orangey-yellowy blonde, (I think my natural hair has reddish tones as dyes always fade orangey).

I decided to ignore my hairdresser (a mistake, I was a stupid teenager) and I bought 30 vol bleaches to lighten and a box dye (shades described as light ash blonde) and I got a nice colour each time. I had to keep doing the roots obviously as they were dark, but the big issue is the colour fading after a week. I'd be happy for a week and then I'd wear a hood over my head the other weeks until I dyed again.

I'm now at the point where my hair is so damaged, there are little bits of hair sticking up all over the place and it feels pretty rough.

I'm letting it grow out for a couple of months so I have some new hair to work with and then I'm going to get a short pixie cut so get rid of my terrible ends. What I really want help with is managing the colour. I want to stay light blonde, I don't feel confident in darker tones. I also want it to be ash blonde but it's difficult to maintain with my hair always fading to brassy tones really quickly. I'm using coconut oil masks at the moment but I find that it seems to fade the colour more, I also wear a shower cap when I shower and wash my hair separately in colder water.

Any tips on how I can maintain the colour for longer with minimal damage would be amazing and greatly appreciated. Also advice on what kind of routine I should take with dying and the products I should use? Because I don't really know what I'm doing. Thanks a lot!

..I also wanted to add that purple shampoos and conditioners don't work on my hair. I've tried 'Touch of Silver', 'Shimmer Lights' and 'Fudge clean blonde' and all have left my hair with patches of purple at the ends, and the top of my head still brassy. I'm not sure whether this has something to do with the damage of my hair?

Thanks again! *thx*