Advice on dyeing own hair

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For years I had crap hair. Then I found a great hair dresser I have naturally mousy hair and have half s head of highlights in two different blondish shades every three months. It costs £90 a time which I can ill afford, but it looks nice.

I've got quite a lot of grey now and I think I'd like to go quite a bit lighter. Blonde all over. I'm considering doing it myself. Is this a terrible idea? My previous experiences are limited to sun in in the 90s which was never a success

My worries are that it may not be a good idea over previously highlighted hair. My hair tends towards being dry anyway and the grey. Might it come out an odd shade. I'm also worried that if I do, when I go for a trim she'll bollock me