couple of questions

Can you post a photo of your hair now?

How light is it? You normally need toned to white hair (starting as almost white, inside of a banana colour) to get pastels. It might not be as light as you think, especially if it's patchy.

Pastels are tricky, they show up every flaw in your base. Hair that has been bleached multiple times will also have areas that vary in porosity. Protein treatments can help even out the porosity. (Coconut oil doesn't add protein BTW it just helps to prevent protein loss)
How exactly are you making and applying your pastel mix?
Ideally you should wash with shampoo to remove any product build up, don't use conditioner. Apply evenly and thickly with a tint brush. I find i get better results using a silicone free conditioner to dilute. I prefer to apply on to dry hair, but some people get better results over damp hair.
If it is fading to blue/green, adding a little pink to your regular conditioner will help to keep it lilac for longer.

The pictures don't really show how light it is, but you can see the unevenness. It's only been washed once since the last time I dyed, the uneveness will become a lot more obvious after a couple more washes. It's possible that some pieces around the middle of the hair are too dark (I know one is, cause I missed it during bleaching once  *whistle*), but I considered the rest 'inside of a banana peel'-yellow.
I have been using conditioners and masks with keratin and other kinds of protein for the last 6 weeks, but they have not helped so far. I've also done coconut oil treatments, but those just made my hair feel really brittle and dry.
Before dying I use a clarifying shampoo, then put the (sometimes diluted) dye on damp or dry hair (I've tried both). I've diluted both with a normal conditioner and a silicone free one, it didn't make a difference.

Sorry for any mistakes, I'm not a native speaker. And please don't mind the mess, I'm reorganising.

Have you tried just reapplying the dye after the first wash? That's not as bad as I was imaging and the parts not purple are looking like they're on the way. They must have just been more yellwo and so toned to grey/blue/greenish.

If the protein isn't helping, just stop. I have the same problem and I only use coconut oil when bleaching (more for my scalp than anything!) and kpak reconstructor right after the bleach. I now just use argan oil and olive oil and it's a lot better.

It looks to me like the pieces that are more grey were more yellow, and the lilac dye has toned them to silver rather than dyeing them lilac.
What dye are you using? I didn't have great results with Directions lilac, it faded really quickly, so a different dye may be the solution. I am currently using a mix of Special Effects Deep Purple and Blue Velvet heavily diluted, it is much better IME.
If you don't want to lighten your base further you will have to re-dye the grey parts with a darker lilac mix, (often layering the colour helps with pastels) try a pinker mix (warmer shades are more forgiving on less than perfect bases) or go for a darker shade all over.

I have tried redying the grey bits, but it didnt help much. Even when I'm applying the dye, it just doesn't show up as much on some parts, it barely looks purple on the hair. I thought the colours in my hair were just too warm at first, but I have used a silver shampoo and it's been dyed lilac 3 or 4 times now, which I assume would have toned the hair enough. I will try again though. I also feel like I am using a lot of dye, could this be because of the porosity or is it just because I have a lot of very fine hair?

I use directions because it's the cheapest and easiest to get (even cheaper than crazy colours). I would totally buy a darker, more expensive dye if I could dilute it and use it, but diluted dyes sadly do not seem to take at all, except near the roots (the normal dye takes better there as well, as you can see in the pictures). I really don't get why diluted dyes won't work for me.

Also gonna repeat this question: Does anyone know a good pastel blue or green dye? Something like bleach london's blullini or washed up mermaid: very pastel, not too bright. I can't get bleach london in my country, Directions would be easiest for me.

*thx*

The closest to a pastel blue or green that Directions make is lagoon blue, but I find it quite a finicky dye that needs a pale and even base. If you don't want to dilute you will be very limited with Directions, i have always used their deep colousr diluted with great results. Atlantic blue, midnight blue and alpine green all make lovely sky blue/aqua shades when diluted.
Pravana have a range of pastels, they are very pale so possibly wouldnt work over your base.

Thank you for the recommendations!
Does anyone have any idea why diluted dyes won't take? Or more tips on how to get them to work properly? I might do a bleach bath next time I touch up my roots and try again.

You can get foaming issues with some conditioners, which makes application difficult and can create uneven results. Applying with a brush helps, as does mixing in advance so the mix has time to settle before use.

okay, thanks!

I think you need more pigment for the lilac to take evenly. As the others said, its toning your hair as those parts were more yellow. You can add a tiny bit of purple dye to the lilac on those parts to even it out.

Alpine green has a nice blue/green pastel fade but i have never tried diluting it for a pastel.