As someone who was diagnosed with ADHD combined type as a child but who has also worked in a school with children with SEN it is definitely essential to get a diagnosis. Disabled students services must be sick of the amount of people who walk into their office saying they have bad concentration, might have ADHD or even (yes I have heard this one) my hand gets tired when I write for too long, do I have dyspraxia. its sounds heartless of me to say this but it's true, many people hear of these disorders, think oh I have bad concentration I must have this and don't realise just how severe it should be to warrant a diagnosis. Many average people can't concentrate for 2 hours straight.
Having a diagnosis will help you as it will set you apart as someone who definitely has severe enough symptoms to require help but also because in the report you receive the psychologist will outline exactly where your difficulties are and strategies you and your university can use to help these.
The main issue is it can be very hard to get diagnosed at this age and it is even harder still to receive medication for ADHD as an adult if you weren't on it as a child (strattera being the exception). This is because all the stimulant medicines such as concerta are not licensed for use in adults who did not have it prescribed as children. Also due to the fact that many doctors still believe you should grow out of ADHD by this age, that if you really had it you would have been diagnosed as a child or that due to your age you could simply be looking the medicine for it which is known to be abused. These misconceptions may be false but they can still affect the chance of your doctor taking your request seriously. If you have any evidence from your childhood that can back up you have had these problems from a young age and they have persisted and are severe enough to warrant a diagnosis then you may hep your case. This could be in the form of old school reports, social services letter, previous inquiries made to your GP (maybe your mum took you when you were a child concerned about your concentration) or an educational psychology report.
Either way a diagnosis is essential for getting through uni as you will need it to access any help you can get but it may be difficult to be taken seriously when looking for it, my boyfriend tried to get a diagnosis aged 18 and the doctor would not take him seriously, he produced an educational psychology report from primary school that stated he had problems with attention span and motor skills and he got referred to an adult autism centre who sent him back to the gp because they didn't deal with ADHD and the gp then said he didn't know where else to send him. He then gave up trying to get help, failed school and works in a bar despite probably genuinely have ADD so be prepared to fight your case, don't be put off when they say they don't know where to send you or that you're too old. Bring all the evidence to support you that you can, possibly even your mum as her word on your difficulties may help. Not because they think you are lying but because your mum was older and will remember more than you and may be able to identify areas you didn't even notice as a problem. Honestly looking back on my own childhood I would say other than a few mishaps I wasn't that bad but my mum will tell a whole different story, starting with me breaking her tv, dvd player, video player and oil lamp in one kick of a ball and ending with that week we had 2 different trips to A&E for a busted lip.