Dry itchy could be lichen sclerosis? I wondered this too tbh you say they ruled it out - how? Have you had a biopsy? - it's really the only definitive test - Have you been advised to use a barrier cream like sudocrem?
"Sensitive" detergents and fabric conditioners aren't necessarily the answer - dd and I are allergic to them.
Have you been tested for stis and bv? Good to rule out these things I think.
Have you changed or started new meds? Even oral ones can have weird effects. Especially things like hormonal meds.
Are you using a mirena coil or even a copper coil? They can cause odd reactions too
Have you a sexual partner and are they also being treated for potential causes or are you abstaining from genital contact for periods of time? (No need to actually answer this but something for you to consider - if it's something infectious you could just be passing between you regularly, loads of these affect women more than men, with men often being asymptomatic)
If you use condoms could be a latex allergy or allergy to the spermicide (I have this problem) or if you use vibrators sometimes the material they are made from can be an issue or even the dyes if they're bright colours. I had a bright pink one once that was no good for this reason 
On loo roll - is it coloured? Or scented? (Should be banned imo awful stuff) That can be an irritant for some people
Psoriasis isn't always red. My friend has it and it's more white or cream coloured on her remember "typical" presentation doesn't mean "always", she is mixed race and on the darker side of that.
Implant could be factor - does that time in for you? Maybe started a few weeks after getting implant?
GPs are pretty shit on gynae and dermatology ime
Do you know what they're actually testing swabs for? No point them repeatedly testing for the same things and not finding the actual cause if they're not testing for it
@mcclucky there are agenda based reasons why gps very rarely refer patients to specialists it's appalling