I'm sorry, but some of your post is ringing alarm bells for me...
If you vape 18mg, then under the "nicotine law" otherwise known as the TPD in the EU and the TRPR in the UK, the only way that law affects you is that it limits you to 10ml bottles, and that applies to any strength of nicotine, not just high nicotine.
In all the vape shops and online vendors, 18mg juice is freely available, so you do *not* have to mix.
Perhaps you have a PG intolerance, or an allergy to something in the setup you never changed from.
I would like to clarify my post, as I feel it may have been misinterpreted.
I was not questioning the motives of the OP in any way, shape or form. The "alarm bells" comment was in reference to the misunderstanding of the "nicotine laws" meaning the OP has been DIYing their own juice when there was absolutely no need to, and that they haven't tried changing any of their ingredients to see if they have an intolerance or allergy.
If someone told them that in order to continue vaping 18mg they have to mix their own juice, they have been completely misled.
The fact this has led them to mixing their own, using the same recipe for years, and possibly suffering side effects is what was setting alarm bells off. It's something in their setup.
They haven't tried different flavours, different PG/VG ratios, or reducing their nic.
It could be a combination of an intolerance to PG or VG (which I believe might cause dry eye, as it can certainly cause dry skin for certain people), and perhaps they are on too high a nic content too - most of us only stay on high nic when quitting tobacco, and then gradually reduce our nic over time.
My post was a simple statement that the fact they have been misled about the TPD/TRPR and have been mixing their own juice in exactly the same way is setting off alarm bells for me - I wasn't being defensive in any way, I said it could be something in their mix that is causing the problem.
It is unfortunate that they never tried changing things up to see if something in their mix was giving them these symptoms. If only they had spotted all the 18mg eliquids in different flavours and PG/VG ratios on sale in supermarkets, chain stores, newsagents and garages, or tried reducing their nic content, they might have been able to isolate the problem.