Hi,
Sorry, this is a long post!
I've not made a post before, but have read these forums for years, and always considered joining to ask for help but been a bit scared. It's now reached the point where I just feel I need an objective outsider's opinion. I hope this is an okay place to ask this 
I had depression for most of my late teens (now early twenties). It took a while to go to the doctor about it because I was just nervous they'd think I was putting it on. When I finally went, it did feel like a load off, but in my mind I still felt as though I was pestering the doctor, who had probably dealt with 70 more severe issues that morning.
I had no interest in life, each morning I felt as though I would've rather not woken up. I constantly felt as though I was putting on a front to others, even my much-loved partner and parents. I felt my entire personality was fake, and despite attempting to show happiness, I never felt it. I cried almost daily, sometimes with no no apparent reason, sometimes at just the overwhelming feeling of life.
I was prescribed citalopram 15mg. I managed to stick with this for almost 7 months but the nightmares were almost unbearable. Nightly dreams about my partner, my parents or my pets dying. Torture and murder. Incredibly upsetting and I couldn't continue. After taking so much effort to ever go to the doctor, and in my frustration, I just totally gave up with medication, naively thinking things would get better on their own, and perhaps that the nightmares were a sign medication wasn't suited to me (I know see how daft I was but I think this was my then-justification).
Since then I have dealt with the depression alone with varying degrees of success. Some months I feel numb to it, and others I am consumed by it every day. I have been to university and am currently half way through my masters. I have hated the entire experience. The self-doubt and self-loathing of continually producing work I have no faith in has been crushing, and totally ruined an entire field for me (despite getting good results). If I felt I could get another opportunity at my MA I am sure I would drop out now, but I am financially invested and feel unable to let my parents down by quitting something they are so visibly proud of.
After that huge waffle for context, my main problem is:
I really don't think I'll be able to continue my course if things don't change. I'm barely sleeping, I can't concentrate on my work and have none of the motivation required to do it justice. I have no desire to look after myself (food or personal appearance) through my constant low mood. My sex drive is non-existent. I am incredibly introverted and happy to have minimal contact with others, but find any contact whatsoever at the moment is totally exhausting and I will avoid ever leaving the house if possible (I work PT from home). I do not feel suicidal but have no desire to live such a joyless and overwhelming life.
If I go back to the doctors, is it likely I will have to go through the initial stages of diagnosis again (questionnaire which I was required to take two weeks over, asking general questions about mood). Or will it be understood that as this is a pre-existing issue I am aware of what is happening to me? What are my other options in terms of medication? I have Googled, but any extra info would really help.
I have endometriosis which in itself has also been hard to deal with, and while I haven't been back to the doctor regarding depression I have been several times for this, and I really dread them thinking I'm wasting their time.
I think I'm just asking for a bit of advice on what to do and how to change what has now become a way of life. Thanks a lot x