Hi everyone,
I'm new here, but I am really stuck and thought this might be a good place to come for advice. I'm definitely going to run out of room so hoping I can continue in the comments.
I am a funded PhD student through one of the large UK research councils on a 4 year programme. I am about 16 months into the main research component of my PhD, and, full time, I would have a year and a half (ish) left.
A bit of background, I completed my undergraduate MSci course at the same uni I am at now. It's a good UK uni with a good reputation for the field I am working in. I wasn't quite sure what to do after my degree, and for a time thought that leaving science behind and trying out another career (another qualification needed) was a good idea. I tried this and got about 3 months into the new course and realised that that career was definitely not for me, mostly due to the current political situation and underfunding. I quit this course and was living with parents at the time, so I ended up with a couple of jobs in retail which I'd worked in before part time, just until I worked out what I wanted to do. It was a huge thing for me to quit and I felt disappointed and like I was letting people down (I'd never quit anything or really struggled like this before)
A friend got in touch and let me know that there was an approaching deadline for PhD candidates at the uni we went to, and said they were applying and thought I may be interested. I decided to apply, as I had enjoyed the lab work and did really well in my masters project, I actually quite enjoyed the writing of the dissertation etc and thought it was worth a shot. I applied, got through the interview and was offered a place.

I was delighted as I always thought I wasn't quite smart enough to do a PhD, and accepted the place. I felt really good about the future, excited about going back into science and although the idea of staying in academia never really appealed to me I was envisioning the doors having a PhD in science would open for me.
I started the course and luckily, got on with my supervisor like a house on fire. I loved the environment, it was a very friendly building in general and there seemed like a lot of support. I heard other students complaining about unsupportive supervisors etc but this never was an issue for me, we got on really well and I always felt supported and listened to. I loved everyone in my lab and was really enjoying learning new lab techniques. I always felt a bit like I wasn't quite as smart as everyone else, but I reassured myself that I would pic it up over time and start to be able to defend my work, talk about my research and join in more with the research-based conversations.
My project is a little random, and is not based on any previous work, it was more of a side prject of something my supervisor thought could be interesting to explore. I was really laid back and easy going about the project and happy to try new things since the overarching topic was very interesting to me and my favourite topic when taught in my undergraduate degree. I felt really excited to be working in this field and any negative not good enough feelings I put to the back of my mind, as I thought, it's because you're new and you'll grow in confidence over time.
Around six months in I had a review with my internal assessor and the "viva" threw me, as I realised I didn't have as good a grasp on my project and relevant literature as I thought.

I came out of the viva feeling very defeated and quite embarrassed at my lack of general subject knowledge. I was a bit down about it but picked myself up and my supervisor and I had lots of talks about why I think it went the way it did and how they could support me going forward. I felt like I'd learn from my mistake and move forward. After all, I still had so much time left and it wasn't a big deal to be underprepared at this stage.
As this all was going off, I had begun getting rather frequent migraines which were beginning to interfere with my life quite severely. I had been back and forth to the doctors, tried cutting out anything and everything that may trigger them. I tried some medication and for a few months after the viva they started easing, perhaps one or two a month instead of a couple a week, and I was able to continue working, gathering a small amount of data. However I had started feeling quite down about the PhD, and the constant feeling of not being good enough. Everyone reassured me this was normal and it would pass. Last summer the migraines came back quite badly, and have not been under control since. At their worst, I was having 2 or even 3 in a week and this was meaning I was missing days of work and falling behind. Due to the lab based nature of my work, missing one day messed up more than just one day's worth of work and I was getting super stressed about the time I was missing, and feeling very guilty for missing time. My supervisor has been very supportive and doesn't blame me at all. We struggled to pinpoint the cause of my migraines and the only trend seemed to be stress as I was always worse when approaching deadlines, presentations etc, and especially my yearly review.

This is a a year after my first review and it was time to meet my internal again. They were made aware of my migraine situation and the stress trigger, and were very understanding and did not push me on questions I couldn't answer which I was very grateful for. However in the viva, they did let me know that the amount of work I had submitted was not enough for the time I had been working and wasn't sufficient. However they were happy for me to continue as I discussed the potential of going part time and/or an extension due to my illness. People did say perhaps I should consider if it's worth continuing the PhD if it is causing me to have these migraines, but I decided to pursue dropping to part time and requesting an extension for sick pay.
This request took around 3 months to sort due to the complexity of the situation but finally I was approved for part time. In this time, I have made very little progress on my project. I now have much longer left to submit my thesis, as my submission time has been doubled.
I have always battled with my mental health, my anxiety has been extra bad since my migraines started and I knew this was impacting the migraines or vice versa. However, in the last 6mos-1year, I have felt very low and even depressed. I often have feelings like whats the point, and don't want to get out of bed and go to uni. I have no passion left for my project although I love the topic I work in. I now feel in a cycle of feeling awful, getting stressed, causing myself migraines and/or tension headaches and missing work, getting stressed for being behind etc. I thought that going part time would help me to feel less pressure and more time for myself to do things that make me happy etc.

But I've now returned to work after Christmas and the way I feel is at rock bottom. I am not sleeping properly, my migraines have come back (even though I was fine over the Xmas break) and my self esteem is on the floor. I don't feel like myself, I am detached and just miserable.
I am seriously thinking of quitting, but I feel like I will be letting myself and everyone else down (yet again). I worry that I am just a quitter and I will never settle in anything, and I know although my family just wants me to be happy, they will be disappointed that I am giving up and yet something else. I have NO CLUE what I want to do with my life and my career, but I know I don't want to stay in academia. I don't even know that I want to stay in science or even would need a PhD in my future.
I don't want to regret such a fantastic opportunity that I know people would be so jealous of, im funded, paid (stipend), have the nicest supervisor ever (a rarity I know), an interesting (if vague) project and I do enjoy the lab work.
I feel so far away from completing the PhD and the thought of slogging away for another (now) 3 years is just painful. I have nowhere near enough to complete, and still don't feel comfortable talking and/or presenting my work. I feel like a fraud and like I genuinely am not good enough (I know everyone says that, but I honestly don't, and I don't think it's just imposter syndrome). But I don't know what else to do, and I know my supervisor would be so disappointed in me after all the help and support they have given me in the last couple of years.
My partner says I need to at least give the part time thing a chance, but I couldn't even bring myself to leave the house this morning. Please help!!