accused of harassment after requesting the deletion of my records from a clinic

I'm curious, if it's a private clinic, why does it matter what diagnosis they have written down? Those records aren't visible to the NHS and they can't share them with your GP unless you have given them explicit permission to. Would them removing the diagnosis really solve the issue with your BF?
Regrading your chances of winning in court, I think even appropriately qualified professionals would struggle to give you an answer without significantly more detail than you have given here. If you can afford it and are desperate to pursue this, might be worth making an appointment with a lawyer to discuss?

i don't care about the nhs. i care about the fact that they maliciously misdiagnosed me for profit and when confronted, they still deny any wrongdoing and won't even change it in my records. it all started right after i stopped attending my regular appointments and later requested to be removed from the patient list - they wouldn't stop calling, and even contacted my boyfriend. i'm an adult. they've fed my boyfriend lies and turned him against me, literally brainwashing him with their lies, making him believe there's something wrong with me, and not their actions.
how could i ever accept something like that? would you? i doubt it. trust me, i tried to just move on and forget it, but i literally can't. it drives me nuts. they're trying to ruin my credibility on purpose
and yeah, that's the plan, maybe i'll give them one more week or something

Unless you were deemed to be a risk to yourself or someone else (in which case, surely they should be calling the police or an ambulance or something rather than your boyfriend?), I'm with you that you should be allowed to stop attending appointments and to remove yourself from the patient list if you wish. And them contacting your boyfriend seems like breaking confidentiality. Don't know the whole story, obviously, but yeah, I'd be pretty annoyed too in your situation.