Haven't read the whole thread sorry.I have got a cold and my attention is even worse than usual.
Firstly HUGE congratulations for getting where you are. That is a PHENOMENAL achievement and you should be seriously, seriously proud. I've never managed to get past GCSEs to my utter utter shame and all my teachers'/parents' etc bemusement. Not for a lack of attempts either.
GO TO DISABILITY SERVICES. And your personal tutor. Tell them the problems that you're having, take some info on executive functioning difficulties (this is the main thing, I find, that gets in the way of stuff like education and life admin).
Extra time is pointless. Extra time is what they throw at you because they have no idea what support to provide you. You give somebody with ADHD extra time and they're going to spend the extra time daydreaming and staring at the clock - if anything with ADHD you need less time in order to give you a sense of urgency :o - Not my words BTW - from the wonderful Dr. Russell Barkley - have a listen to his lectures on youtube:
(Ignore the fact that says for parents - it's for anyone.) I couldn't focus on just the audio so I played it in segments while tidying up or putting away clothes. It's really, really good.
First rule of having something like ADHD: Do not expect anybody else to understand what ADHD is or what difficulties it presents for you, unless they are an expert in the disorder and even when they are, they are not an expert on you. Everyone has a slightly different case, and the sign of a good doctor/teacher/support person is when they are open to listening and taking on board what you ask for and helping in that sense. It's not reasonable or efficient to expect every teacher/support worker/etc to be an expert in every single disorder, whether that's dyslexia, ADHD, autism, etc. You need to be the expert on it so that you can communicate clearly what it is that you need from them.
For example, you probably don't need extra time in exams. But you might need to spread units out so that the degree takes twice as long overall. That's a good accommodation and one which works well with being a single parent. It's how I got through my first year of uni (and then forgot to go to the exams - long story). You very possibly could do with somebody who is good at pacing to sit with you and an academic calendar (or digital calendar, but I found a wall one much more visual and helpful), go through all of the assignment due dates and block out however many weeks they would usually recommend spending on them, multiplied by 1.5 for you, 2 if any overlap, before each deadline. Bear in mind that if you spread out the time too much, you'll be starting before you've attended the relevant lectures and seminars, which makes it really difficult if not impossible - this is one reason to spread out to part time doing half the number of usual units. Neurotypical students can typically cope (if barely!) with 3-4 assignments ongoing at the same time but with ADHD this is an organisational not to mention a focusing nightmare.
Once you've got your overview of the time you need to spend on things you can then block out the number of evenings, naptimes, nursery sessions (whenever you usually work). REMIND YOURSELF to keep a few in reserve to account for a poorly child, an evening when you yourself are exhausted and fall asleep at 7pm, and so on. So if you can usually complete an assignment in 8 "work blocks" you must allow 10-12 - just in case.
I would recommend not to ask your tutor for flexibility with deadlines. Often we "need" the stress of a deadline to actually get something done. Possibly though ask him/her for a (limited!) number of "free passes" for extensions because there will be times when you need one, but you need to sort of kid yourself that they won't be easily got so that you don't fall back and rely on them.
I don't know if all of the above is still valid for a postgrad - sorry 
but those are the kinds of things I would start to think about along the lines for. Also writing yourself a sort of "instruction manual" as in how to write an essay (e.g. step 1: Collate all relevant lecture notes, step 2: Access all relevant readings, etc). Oh, also, electronic versions of readings were a lifesaver for me. If this isn't currently accessible to you, ask for that - your tutors might be able to request digitisation of the relevant chapters. This is phenomenally useful, partly because you can use tools such as "find in this page" but also because you can access them anywhere and don't need to worry about physical books/papers, keeping them away from the toddler, light sources to read, or LATE FEES (which are known as the ADHD tax in my house).
Re medication: If you're on an extended release type, you could try taking it 2-3 hours later in the morning. You will lose a bit of productivity at work, but gain some more focus and clarity in the evenings. Or is it possible to change your schedule so that you take work into uni and work on it in the library? Even one session a week of this could be hugely productive. If you are on an instant release medication, you can mimic an extended release by cutting a tablet in half and taking it in two parts. You could then take three on days that you really need to study, as long as you find it doesn't affect your sleep too badly. You could also ask about switching from instant release to extended release or vice versa - as this wouldn't actually be a change in medication, it's possible your GP would be open to discussing it or referring you to somebody who can on the NHS.