Hey okay so I've commented a couple of times already. Sorry for the spamming. Your story get deeper and deeper and makes more and more sense as you keep commenting though. To start with it sounded like you HAD medical problems that affected your self care and that you are now fully capable physically, but not in the habit of good self care now.
But the more you comment the more it sounds like you are still affected by medical issues and will continue to be for the rest of your life to at least some extent. Looking back it seems like I also missed some comments in you OP that suggested you still have some physical issues.
It sounds to me like you're just starting to get a feel for adult life and the wider world and how that relates to your situation. That must be a really confusing thing so I'm going to try to cover a few things. Some of it might be obvious or irrelevant or whatever so please just skip and roll your eyes at me if that does happen. But life doesn't always make much sense when you've started off experiencing things so differently so I'm going to try to explain things as much as possible just in case there is something you've missed out on (or maybe other people reading this even).
Disability isn't what a lot of people think it is. It's not like you apply to the government or your doctor and get a certificate that says "this person is disabled". Disability has different levels (like social and legal) so depending on the situation somebody might be disabled some of the time, but not other. It's not totally black and white and for the most part it's up to the individual how they identify.
As far as getting benefits, support at work etc goes there are boxes you need to tick, but that's not really weather you are disabled or not, it's weather you are disabled in the right way or to the right extent. So for example someone in a wheelchair could be physically disabled, but they might be able to manage that perfectly fine themselves so wouldn't ask for support at work because they know they can manage it. Or somebody might have difficulty with certain every day things enough to be beyond "normal", but not enough that they would need additional support like benefits.
You can also be unwell, but not quite to the extent that you would consider yourself disabled. You might look and feel pretty normal, but there are certain things you would need support with and in those situations you would fall under the disabled catagory.
It's all a bit of a mess and basically as long as you fall somewhere in the disability definition (you can find the actual wording on the .gov website but it's basically that you have long term symptoms that significantly affect day to day activities) you can choose when to refer to yourself as disabled, when to ask for support etc. You might not always be eligible for the support, but that identification of yourself as disabled isn't decided by if you are eligible for support, it's if you fit that definition of disability.
From what you've said it sounds like you're somewhere on that scale. It sounds like you have significant weakness in your arms (and potentially other parts of your body?) that mean things like washing your hair are difficult. Thigns like washing your hair, tieing shoe laces, boiling pasta in a pan, wiping your bum, putting clothes into a washing machine etc are some basic day to day activities and it sounds like they probably aren't as possible for you as they should be.
If that is the case then you are somewhere in that area where you could reasonably call yourself disabled. You can also use words like unwell, ill, impaired etc if you prefer them. Really it's a personal choice.
If you often need your mum to help with those sorts of things then you might also find you are entitled to some form of support. That might be thigs like physio to help you learn to do them by yourself, a carer to come help you with those thigns so you're not so reliant on mum or equipment that helps you or another person do those things more easily.
It might be worth talking to your GP about an assessment to see what sort of support you could benefit from. If you ever go to uni you can also apply for something called DSA which helps disabled students (and students who just have some extra needs) access education without their issues getting in the way as much.
You might also benefit from joining a support group. There are various types but basically they are all people in similar situations discuss thigns. In your case that might mean joining a group online for young people with chronic illnesses and discussing being independent when you've been reliant on a parent for so long, how to tackle personal hygiene, how to date, funny memes related to your situation etc.
As far as habits, life experience etc go and thigns like having somebody to tell you when you smell... That might be a pretty big issue for you now but there is definitely hope. You've had a kind of slow start cos there's been a lot of hurdles, but humans form habits it's like this natural instinct for us. Even as adults we can form new habits and things that used to be hard can become very simple. If you spend some time making a real point of doing things you'll eventually get used to it.
Like for instance you can start cleaning yourself (be that bath, shower, wet wipes etc) the same day each week. Set reminders on your phone, do it the same time and try to set it around a routine you already have (like going to bed or waking up). It will be hard to start with and feel unnatural but eventually your body will learn that Wednesday night is bath night.
Or if a full body clean is a bit much for you you can start with washing your pits in the sink or something. Whatever is do-able for you. Small steps are still good.
You can work out a few milestones you want to reach and work towards them. You could even ask mum, siblings, GP, teachers, friends or whoever else to help with that. Those goals are totally up to you but they might be things like regular washing, making a basic meal like a sandwich, getting a bus, taking your medication, washing your dishes after a meal or even changing your pants each day. Whatever manageable thing you want to learn to do.
Start with one pretty simple thing and get yourself a bit more used to reminding yourself to do stuff and take it from there. Go at whatever pace is right for you. Maybe each night you can ask your mum to leave pants on your door handle so then in the morning you'll be able to remember to put them on. Then start putting them out yourself each night or finding them in your wardrobe each morning.
Try to get your mum on board with helping teach you to be more independent. She'll still be carigg for you and doing her motherly duty, but it will be helping you to grow too instead of keeping you in this awkward not quite grown up state.
Anyway sorry for all the posts. I hope its not been too annoying for you and that what I've said makes sense. If you do want to find a support group Facebook can be a good place. Most groups are private so only members can see posts and you can always create a separate account that doesn't have your full real name so you can post more anonymously.
I'll stop rambling now cos I've gone quite far away from your title question and I don't want to be a total annoyance for you. I just feel like my first comment was kinda unhelpful after reading a bit more of your situation so I didn't want to leave it like that.