Hair won't take the dye

[seabreezewavingtrees]

I have dark blonde hair and dyed it a rich deep brown permanent colour (4 I think) last July.
I really like the colour when my hair's straight, and I do flat iron it quite often. But when it's curly I feel like it lacks the highlights that were there naturally and it being all one colour looks too much. I think the colour is too strong. I know people fairer than me with brown or black hair and they look lovely so I'm not sure why I can't pull it off, but when it's curly and all around my face I look washed out. (Not had my colours done but I'm mixed race (black/white) green eyes, quite pink in the cheeks, nose, forehead)
I've tried to dye it back to something closer to my natural shade using Garnier Nutrisse but it's made no difference. If I look very carefully at the roots in good daylight there's a subtle difference in the shade of brown where they've started to take, but barely noticeable. I was going for a dark blonde. I tried it again, and left it on for 10 mins extra and still nothing.
I'm on mat leave and can't afford a hairdresser.
How do I get my hair back to its old colour? Scared to strip it in case it falls out and can such old dye be stripped now?
Hair type = natural, 3b/3c curly. (Like Angela Griffin)

Unfortunately, you can only dye light hair dark but not dark hair back to light.....you either need to wait for it to grow out or bleach it and then dye it to the shade you want...but I don't think bleaching it is good for your hair.

Yep, you'll have to bleach it. Don't risk it yourself. Could you look at having it done at a hairdressing college? They're very cheap and heavily supervised.

As a guide our college charges from £10 for a full head colour.

Thank you, I'm such a novice I didn't know that you could only go from light to dark. I'm googling hairdressing college now. Thank you

You shouldn't need to bleach it. You should be ok to strip the colour with something like this. It's not great for the condition but it shouldn't make it fall out.
You can't guarantee what colour it'll come out so it would be good to be prepared to even out any patchiness with a semi permanent.